[TASK] EXT:form - add email finishers explanations

Reduce confusion caused by the naming of the email finishers and
add some explanations to the recipient address/name and
sender address/name properties.

Resolves: #81798
Releases: master, 8.7
Change-Id: I0dc12e2983d7cb1204bdfc9709265c8c034a58f6
Reviewed-on: https://review.typo3.org/53407


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>