[DOCS] Mention allowed classes in f:fluid image attribute docs

Releases: master
Resolves: #95208
Change-Id: I852fbdc1d34afb819798cb93b2bb2f2b05e7a082
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71051


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>