[Doc] Add documentation for masks in tensor specs

[kurtamohler]

Description

Add documentation for masks in the four *DiscreteTensorSpecs.

Motivation and Context

close #2280

• [x] Someone has raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (update in the documentation)
• [ ] Example (update in the folder of examples)

Checklist

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• [x] I have read the CONTRIBUTION guide (required)
• [x] My change requires a change to the documentation.
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [x] I have updated the documentation accordingly.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2289

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:heavy_exclamation_mark: 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Too many GPU instances requested - any *.nvidia.gpu and ephemeral instances should experience significant queue times

:x: 2 New Failures, 4 Pending, 16 Unrelated Failures

As of commit b0cc6f5055f47a72c4fe6c7fca9835143206d4d4 with merge base 8e43ac8f48b9b6aead0b693674e8176bbda8221b ():

NEW FAILURES - The following jobs have failed:

* [Continuous Benchmark (PR) / CPU Pytest benchmark](https://hud.pytorch.org/pr/pytorch/rl/2289#27312250784) ([gh](https://github.com/pytorch/rl/actions/runs/9888406644/job/27312250784)) `Workflow failed! Resource not accessible by integration` * [Continuous Benchmark (PR) / GPU Pytest benchmark](https://hud.pytorch.org/pr/pytorch/rl/2289#27312251384) ([gh](https://github.com/pytorch/rl/actions/runs/9888406644/job/27312251384)) `Workflow failed! Resource not accessible by integration`

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

* [Examples Tests on Linux / tests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312252396) ([gh](https://github.com/pytorch/rl/actions/runs/9888406656/job/27312252396)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Generate documentation / build-docs (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312252978) ([gh](https://github.com/pytorch/rl/actions/runs/9888406675/job/27312252978)) ([similar failure](https://hud.pytorch.org/pytorch/rl/commit/b0cc6f5055f47a72c4fe6c7fca9835143206d4d4#27312267346)) * [Habitat Tests on Linux / tests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312252662) ([gh](https://github.com/pytorch/rl/actions/runs/9888406686/job/27312252662)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Libs Tests on Linux / unittests-gym (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312267346) ([gh](https://github.com/pytorch/rl/actions/runs/9888406669/job/27312267346)) (detected as infra flaky with no log or failing log classifier) * [Libs Tests on Linux / unittests-sklearn (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312268184) ([gh](https://github.com/pytorch/rl/actions/runs/9888406669/job/27312268184)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Lint / c-source / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312252416) ([gh](https://github.com/pytorch/rl/actions/runs/9888406682/job/27312252416)) ([similar failure](https://hud.pytorch.org/pytorch/rl/commit/b0cc6f5055f47a72c4fe6c7fca9835143206d4d4#27312267346)) * [Lint / python-source-and-configs / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312253408) ([gh](https://github.com/pytorch/rl/actions/runs/9888406682/job/27312253408)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [RLHF Tests on Linux / unittests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312251156) ([gh](https://github.com/pytorch/rl/actions/runs/9888406678/job/27312251156)) (detected as infra flaky with no log or failing log classifier) * [Unit-tests on Linux / tests-cpu (3.10) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312254595) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312254595)) (detected as infra flaky with no log or failing log classifier) * [Unit-tests on Linux / tests-cpu (3.11) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312254991) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312254991)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-cpu (3.8) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312255728) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312255728)) (detected as infra flaky with no log or failing log classifier) * [Unit-tests on Linux / tests-cpu (3.9) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312256353) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312256353)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-gpu (3.10, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312257719) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312257719)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312258218) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312258218)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312258988) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312258988)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-stable-gpu (3.10, 11.8) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2289#27312260338) ([gh](https://github.com/pytorch/rl/actions/runs/9888406674/job/27312260338)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128`

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