docs: Update hybrid offer invariant comment

[Kassaking7]

High Level Overview of Change

This PR (#6716, to develop) contains confusing comment: The original code doesn't have any size check on sfAdditionalBooks

Rephrase the comment to have better readability. The comments follow the comment from this PR(#6945, to staging-313)

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 81.9%. Comparing base (7a449ed) to head (b30fac6).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7007     +/-   ##
=========================================
- Coverage     81.9%   81.9%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        76383   76383             
  Branches      7530    7530             
=========================================
- Hits         62522   62520      -2     
- Misses       13861   13863      +2     

Files with missing lines	Coverage Δ
.../libxrpl/ledger/helpers/PermissionedDEXHelpers.cpp	96.6% <ø> (ø)

... and 1 file with indirect coverage changes

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