fix chain_prop and compact_prop docs

[OriolAbril]

Description

Fix docstrings that were outdated and add the defaults in description.

Checklist

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2176.org.readthedocs.build/en/2176/

[symeneses]

Thanks for your contribution!

[codecov]

Codecov Report

Merging #2176 (0528078) into main (4834f3b) will increase coverage by 0.82%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2176      +/-   ##
==========================================
+ Coverage   88.17%   88.99%   +0.82%     
==========================================
  Files         121      121              
  Lines       12700    12875     +175     
==========================================
+ Hits        11198    11458     +260     
+ Misses       1502     1417      -85     

Impacted Files	Coverage Δ
arviz/plots/traceplot.py	95.34% <ø> (ø)
arviz/data/converters.py	55.43% <0.00%> (-11.69%)	⬇️
arviz/data/inference_data.py	72.50% <0.00%> (-10.42%)	⬇️
arviz/data/__init__.py	100.00% <0.00%> (ø)
arviz/data/io_emcee.py	98.29% <0.00%> (+5.12%)	⬆️
arviz/data/io_pystan.py	96.89% <0.00%> (+36.19%)	⬆️
arviz/data/io_beanmachine.py	86.27% <0.00%> (+48.43%)	⬆️

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