[TASK] Document variables assigned to mail templates

Resolves: #89730
Releases: master, 9.5
Change-Id: I1754168d86cd9d69d866accee9fc042f8e967490
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62363


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