[TASK] EXT:form - Add documentation

Resolves: #80198
Releases: master
Change-Id: Iaad2a96c2b1d7413220521dc188b09700110e391
Reviewed-on: https://review.typo3.org/52485


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>