[DOCS] Document form finishers for editors

A new chapter for editors regarding form finishers has
been added. While being on it a small typo in the form
editor has been fixed.

Resolves: #97026
Releases: main
Change-Id: I94b662d9da45cde6f4c6c4c81161a33294241454
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75002


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