[DOCS] Remove superfluous namespace within form documentation

Removes the vendor namespace (TYPO3.CMS.Form) within form
documentation. This is a follow-up to #97517 and applies
these changes to the form documentation.

Resolves: #97520
Related: #97517
Releases: main
Change-Id: Id3a98df586a5ee5660fa1d113645421ef37bef90
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74493


Tested-by: Elias Häußler <e.haeussler@familie-redlich.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Elias Häußler <e.haeussler@familie-redlich.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>