[DOCS] Fix warnings in form documentation

This patch fixes all remaining warnings in the form
documentation.

Resolves: #97004
Releases: main, 11.5
Change-Id: I185bbc47b9c6f5b037a1673225c33e40fcb88635
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73672


Tested-by: core-ci <typo3@b13.com>
Tested-by: Elias Häußler <e.haeussler@familie-redlich.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Elias Häußler <e.haeussler@familie-redlich.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>