MultiIndex Doc

[Giftlin]

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #18169 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18169      +/-   ##
==========================================
+ Coverage    91.4%    91.4%   +<.01%     
==========================================
  Files         163      163              
  Lines       50068    50068              
==========================================
+ Hits        45764    45767       +3     
+ Misses       4304     4301       -3

Flag	Coverage Δ
#multiple	89.21% <ø> (+0.02%)	⬆️
#single	40.33% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <0%> (-0.1%)	⬇️
pandas/plotting/_converter.py	65.2% <0%> (+1.81%)	⬆️

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[jreback]

thanks!