[DOCS] Adjust documentation for Fluid-based templated emails

A feature regarding the subject and localization was missing
in the initial change / docs, so this is now added.

Resolves: #90463
Releases: master
Change-Id: Ic0de6abad93b45e3a176df016304bd3806997d7e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63343


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Benni Mack <benni@typo3.org>