[BUGFIX] Clarify description of FE|sessionDataLifetime

The FE|sessionDataLifetime option is solely used for anonymous
frontend sessions. It does not influence any other session(-data).
Document this clearly.

Resolves: #83168
Releases: master, 8.7
Change-Id: I8b16bc73c6d79b5bc6015b4448c6ec4604bdd780
Reviewed-on: https://review.typo3.org/54871


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Łukasz Uznański <l.uznanski@macopedia.pl>
Tested-by: Łukasz Uznański <l.uznanski@macopedia.pl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>