add extra documentation for buffer_window flag

[mcrauwel]

Description

adding some extra documentation to the buffer window flag

Related Issue(s)

#19038

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

No AI was used :)

[vitess-bot]

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[promptless]

📝 Documentation updates detected!

New suggestion: Clarify buffer-window constraint documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.88%. Comparing base (749fe54) to head (2fb14de).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19039   +/-   ##
=======================================
  Coverage   69.88%   69.88%           
=======================================
  Files        1610     1610           
  Lines      215431   215504   +73     
=======================================
+ Hits       150551   150606   +55     
- Misses      64880    64898   +18     

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[timvaillancourt]

👋 @mcrauwel we also need an update to the flag e2e test here: https://github.com/vitessio/vitess/blob/main/go/flags/endtoend/vtgate.txt. Currently CI is failing