[BUGFIX] Documentation of sessionTimeout

A sessionTimeout <= 0 means that a user will be logged out
instantly after login.

Resolves: #78822
Releases: master, 7.6, 6.2
Change-Id: I1890cb7d52347faab1fa3884eb46c9294d997da3
Reviewed-on: https://review.typo3.org/50797


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>