doc(gh-2107): Improved documentation for Negative Binomial distributions

[Qazalbash]

This PR attempts to address the request for improving documentation for Negative Binomial distributions in #2107.

Fixes #2107.

[fehiepsi]

Awesome, thanks @Qazalbash!

[juanitorduz]

It seems the build docs is failing,

The job failed because of Sphinx docstring formatting errors in numpyro/distributions/truncated.py, specifically in the docstringThe failure in your CI pipeline is due to errors in the formatting of docstrings within the Python file of validate_sample..wrapper. The errors include "Unexpected indentation" and "Literal block ends without a blank line; unexpected unindent." These errors are numpyro/distributions/truncated.py. The logs indicate specific indentation issues flagged by the doc treated asutils package while processing Sphinx documentation:

build failures1. Lines in the decorator @Docstring of numpyro.distributions.util.validate_sample..wrapper:

Line 4: Unexpected indentation because warnings are treated as errors in your docs build.

Can you try make docs if you can reproduce (and fix?) locally?

[Qazalbash]

Can you try make docs if you can reproduce (and fix?) locally?

I will try.

[juanitorduz]

Thanks @Qazalbash !