[TASK] Document further options for mail post processor

* As it's possible to configure the template to use for each instance of
  the post processor, this should be documented.
* Also grouped options regarding part. E.g. email or frontend
* messages.

Resolves: #75754
Releases: master
Change-Id: Ie8cd0702ced989bd4d9306ffe50506981f4c47e4
Reviewed-on: https://review.typo3.org/47739


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>