Fix Timeout's TimeoutGenerator documentation

[peter-csala]

Pull Request

The issue or feature being addressed

2266

Details on the issue fix or feature implementation

• Fixed documentation comment
• Enhanced timeout documentation
• Added test case when TimeoutGenerator is null

Confirm the following

• [x] I started this PR by branching from the head of the default branch
• [x] I have targeted the PR to merge into the default branch
• [x] I have included unit tests for the issue/feature
• [x] I have successfully run a local build

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.33%. Comparing base (1aece4e) to head (31c47b7). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2275 +/- ## ======================================= Coverage 85.33% 85.33% ======================================= Files 313 313 Lines 7444 7444 Branches 1122 1122 ======================================= Hits 6352 6352 Misses 746 746 Partials 346 346 ``` | [Flag](https://app.codecov.io/gh/App-vNext/Polly/pull/2275/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=App-vNext) | Coverage Δ | | |---|---|---| | [linux](https://app.codecov.io/gh/App-vNext/Polly/pull/2275/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=App-vNext) | `85.33% <ø> (ø)` | | | [macos](https://app.codecov.io/gh/App-vNext/Polly/pull/2275/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=App-vNext) | `85.33% <ø> (ø)` | | | [windows](https://app.codecov.io/gh/App-vNext/Polly/pull/2275/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=App-vNext) | `85.33% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=App-vNext#carryforward-flags-in-the-pull-request-comment) to find out more.

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[martincostello]

Thanks again!