docs: Add explanatory comment to checkFee

[yinyiqian1]

Add explanatory comment to checkFee to avoid false alarm detection.

High Level Overview of Change

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.4%. Comparing base (8b986e4) to head (a243a45).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6631     +/-   ##
=========================================
- Coverage     81.4%   81.4%   -0.0%     
=========================================
  Files          997     997             
  Lines        74415   74410      -5     
  Branches      7561    7562      +1     
=========================================
- Hits         60611   60595     -16     
- Misses       13804   13815     +11     

Files with missing lines	Coverage Δ
src/libxrpl/tx/Transactor.cpp	93.2% <ø> (ø)

... and 32 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mvadari]

The PR title should be fixed