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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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.