[release-20.0] Add a note on QueryCacheHits and QueryCacheMisses in the release notes (#16299)

[vitess-bot]

Description

This is a backport of #16299

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.65%. Comparing base (3335bea) to head (fdbcf0b).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16306      +/-   ##
================================================
- Coverage         68.66%   68.65%   -0.01%     
================================================
  Files              1541     1541              
  Lines            197817   197817              
================================================
- Hits             135827   135821       -6     
- Misses            61990    61996       +6     

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