Document Field instance reuse workaround

[lundybernard]

Fixes: #1680

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.55%. Comparing base (812b2a8) to head (63886fa).
Report is 119 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1730      +/-   ##
==========================================
- Coverage   94.28%   93.55%   -0.73%     
==========================================
  Files          91      117      +26     
  Lines        7013     8843    +1830     
==========================================
+ Hits         6612     8273    +1661     
- Misses        401      570     +169     

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[lundybernard]

@cosmicBboy Your update looks good to me.
I'm not sure what to do about the codecov/project failure, it looks like there is an inconsistency in the coverage report which is unrelated to this change.