update docstrings in BetaBinomial class

[saurbhc]

What is this PR about?
Issue #5459 - Update Docstrings for BetaBinomial class

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

Bugfixes / New features

• None

Docs / Maintenance

• Yes, as per the issue Update docstrings for better rendering on website and numpydoc compliance #5459

#DataUmbrellaPyMCSprint
cc: @mariyayb
cc: @reshamas

[michaelosthege]

This looks good!

[reshamas]

@saurbhc Thanks for the PR. If you are working with a pair partner can you add their GitHub handle to the above description?
Example:

cc:  @xxxxx

[codecov]

Codecov Report

Merging #5960 (7e34014) into main (e7d0607) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5960      +/-   ##
==========================================
- Coverage   89.42%   89.40%   -0.02%     
==========================================
  Files          73       73              
  Lines       13292    13292              
==========================================
- Hits        11886    11884       -2     
- Misses       1406     1408       +2     

Impacted Files	Coverage Δ
pymc/distributions/discrete.py	99.21% <ø> (ø)
pymc/util.py	76.04% <0.00%> (-1.20%)	⬇️