Fix extended methods docstrings

[OriolAbril]

Description

Checklist

• [x] Follows official PR format
• [ ] Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• [ ] New features are properly documented (with an example if appropriate)?
• [ ] Includes new or updated tests to cover the new feature
• [x] Code style correct (follows pylint and black guidelines)
• [ ] Changes are listed in changelog

---

:books: Documentation preview :books:: https://arviz--2263.org.readthedocs.build/en/2263/

[codecov[bot]]

Codecov Report

Merging #2263 (31b7960) into main (c6da92e) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 31b7960 differs from pull request most recent head a0e7ddd. Consider uploading reports for the commit a0e7ddd to get more accurate results

@@           Coverage Diff           @@
##             main    #2263   +/-   ##
=======================================
  Coverage   87.89%   87.89%           
=======================================
  Files         121      121           
  Lines       12448    12448           
=======================================
  Hits        10941    10941           
  Misses       1507     1507           

Impacted Files	Coverage Δ
arviz/data/base.py	97.61% <100.00%> (ø)