[TASK] Document validators of form framework

Furthermore, move some documentation around
regarding finishers and validators to have
the information at one central place.

In addition, fix a PHP comment regarding the
MimeType validator.

Resolves: #88212
Releases: master, 9.5
Change-Id: Ifa437294c05d3f9dba16f8bb17ac88a43796595b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60554


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>