[DOCS] Add important RST for changed form framework markup

Resolves: #103392
Related: #103117
Releases: main, 12.4
Change-Id: I97f9c68b6ad1c6b3ded4c70a82dbfe20eb5a9f20
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83456


Tested-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Chris Müller <typo3@krue.ml>
Tested-by: core-ci <typo3@b13.com>