Update multi_model_statistics doc with latest changes (new operators, etc.)

[schlunma]

Description

In #2191, I missed to update parts of the multi-models statistics API doc. This PR takes care of that.

Link to documentation: https://esmvaltool--2321.org.readthedocs.build/projects/ESMValCore/en/2321/api/esmvalcore.preprocessor.html#esmvalcore.preprocessor.multi_model_statistics

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (22e31c6) 0.00% compared to head (da508c0) 93.82%.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #2321       +/-   ##
=========================================
+ Coverage      0   93.82%   +93.82%     
=========================================
  Files         0      240      +240     
  Lines         0    13287    +13287     
=========================================
+ Hits          0    12466    +12466     
- Misses        0      821      +821     

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[valeriupredoi]

thank you, Manu! Clear and concise 🍺