Subclass validator each item docs

[samueldeklund]

Change Summary

Subclass validators do not run when referencing a List field defined in a parent class when each_item=True. Added an example to the docs illustrating this.

Related issue number

#1566

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1592 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1592   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3796      3796           
  Branches       752       752           
=========================================
  Hits          3796      3796           

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[samueldeklund]

@samuelcolvin Not sure if there is anything left for me to do. If so, please let me know and I will work on it.

[samuelcolvin]

thanks so much, sorry for the slow response.