docs: add docstrings for the `data` arguments in the inference functions

[bvdmitri]

This PR adds more information and explanation about the data keyword argument in the inference function as well as for the data and datastream keyword arguments in the rxinference function. Fixes #18 .

[codecov-commenter]

Codecov Report

Base: 79.94% // Head: 80.12% // Increases project coverage by +0.18% :tada:

Coverage data is based on head (e08e2d4) compared to base (395640a). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ========================================== + Coverage 79.94% 80.12% +0.18% ========================================== Files 10 10 Lines 1107 1107 ========================================== + Hits 885 887 +2 + Misses 222 220 -2 ``` | [Impacted Files](https://codecov.io/gh/biaslab/RxInfer.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab) | Coverage Δ | | |---|---|---| | [src/inference.jl](https://codecov.io/gh/biaslab/RxInfer.jl/pull/27/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab#diff-c3JjL2luZmVyZW5jZS5qbA==) | `75.10% <100.00%> (+0.40%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biaslab)

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