Refactor tolerance settings for MS MARCO dense vector regressions

[lintool]

Continuation of #2538

• tweak docs for flat indexes.
• refactor tolerance values for HNSW indexes, calibrate wrt flat index scores.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.07%. Comparing base (9cc2d89) to head (ac56d2c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2541   +/-   ##
=========================================
  Coverage     67.07%   67.07%           
  Complexity     1481     1481           
=========================================
  Files           219      219           
  Lines         12670    12670           
  Branches       1530     1530           
=========================================
  Hits           8499     8499           
  Misses         3644     3644           
  Partials        527      527           

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[lintool]

@MXueguang @tteofili I've verified all results, we should be good to go.