fix: Add description for terLOCKED error

[gregtatcam]

High Level Overview of Change

Add description for terLOCKED error.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (7793b5f) to head (e270721).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6811     +/-   ##
=========================================
- Coverage     81.6%   81.6%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        75979   75979             
  Branches      7637    7637             
=========================================
- Hits         61981   61969     -12     
- Misses       13998   14010     +12     

Files with missing lines	Coverage Δ
src/libxrpl/protocol/TER.cpp	100.0% <ø> (ø)

... and 6 files with indirect coverage changes

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