[TASK] Cleanup form

* Fix old classnames in comments
* Add missing function documentation
* Add \ to DOM* classes

Change-Id: Iec474715af103e0d158e5539b46a31c81ba678ac
Resolves: #45149
Releases: 6.1
Reviewed-on: https://review.typo3.org/18009
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn