[TASK] Add description to allowed_languages field

It might not be clear to users how the allowed_languages field works
by default when setting permissions for a backend user or group.

If no languages are selected from the allowed_languages list,
all languages will be available for the given user or group.

To make this clearer, a description has been added to explain it
straightforwardly.

Resolves: #103757
Releases: main, 12.4
Change-Id: If0c0fd9c998f51a0e44617faae3365f2e02bc82e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84078


Tested-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Thomas Hohn <tho@gyldendal.dk>
Tested-by: Guido Schmechel <guido.schmechel@brandung.de>