[BUGFIX] Document unsetting array property items in form variants

It is actually possible to unset array property items like select
options through the magic "__UNSET" feature of the core ArrayUtility.
This should be documented as such.

Resolves: #89204
Releases: master, 9.5
Change-Id: I26e5ceea8b526cc4624e0004f3344d7e42b49a71
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61764


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>