Revert "Fixed #19303 -- Fixed ModelAdmin.formfield_overrides on fields with choices"

This reverts commit 9d1987d7679165ad3a7c2b713a8a488cc1421905.
Fixed #22690 -- Added a check for proxy models containing fields.

Removed the FieldError raised by ModelBase.__new__ in this case.
Fixed #22748 -- Corrected post_migrate usage example.

Thanks Rudolph for the report.
Fixed #11776 -- Added CSS class for non-field/top of form errors.

Thanks Daniel Pope for the suggestion.
Fixed #22753 -- Documented ability to use a note with the {% comment %} tag.