[SoftDeleteable] Document the $handlePostFlushEvent option

[HypeMC]

Docs for #2958.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.56%. Comparing base (eb53dfc) to head (f39ed75).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2994   +/-   ##
=======================================
  Coverage   78.56%   78.56%           
=======================================
  Files         169      169           
  Lines        8803     8803           
=======================================
  Hits         6916     6916           
  Misses       1887     1887           

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[stof]

@phansys it would be great to have this merged so that the new option is documented.

[phansys]

Thank you @HypeMC!