fix and extend idata.map docs

[OriolAbril]

Description

Fix #1251, maybe #1252 too?

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

[kyleabeauchamp]

LGTM

[kyleabeauchamp]

FWIW, did you confirm that the docs build locally?

[OriolAbril]

FWIW, did you confirm that the docs build locally?

Yes, here is a screenshot

[codecov]

Codecov Report

Merging #1255 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1255   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files          98       98           
  Lines        9715     9715           
=======================================
  Hits         9046     9046           
  Misses        669      669           

Impacted Files	Coverage Δ
arviz/data/inference_data.py	80.11% <ø> (ø)

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