session: update comments for statistics related variables

[Rustin170506]

What problem does this PR solve?

Issue Number: ref #55043

Problem Summary:

What changed and how does it work?

• Updated the comments for some stats-related variables.
• See more at https://tidb-analyze.vercel.app/32?clicks=1

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[Rustin170506]

🔢 Self-check (PR reviewed by myself and ready for feedback.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.9882%. Comparing base (801d5d6) to head (872dc80).
Report is 36 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #55220        +/-   ##
================================================
+ Coverage   72.8365%   74.9882%   +2.1516%     
================================================
  Files          1564       1572         +8     
  Lines        439076     454115     +15039     
================================================
+ Hits         319808     340533     +20725     
+ Misses        99580      92862      -6718     
- Partials      19688      20720      +1032     

Flag	Coverage Δ
integration	49.3577% <ø> (?)
unit	71.7444% <ø> (-0.0894%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.9567% <ø> (ø)
parser	∅ <ø> (∅)
br	51.7748% <ø> (+5.8948%)	⬆️

[Rustin170506]

/retest

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-06 07:54:52.888312081 +0000 UTC m=+338622.755411166: ☑️ agreed by hawkingrei.
• 2024-08-06 08:58:32.702531783 +0000 UTC m=+342442.569630872: ☑️ agreed by AilinKid.

[Rustin170506]

/retest

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, easonn7, hawkingrei

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