[TASK] Better explain when a cookie with a lifetime will be set

A cookie with a lifetime will only be set when the option permalogin
is greater than or equals to 0. If this is not true it will become
a session cookie.

Resolves: #76168
Releases: master,7.6
Change-Id: Ibd5d431107713a118c93295d0f08df721bc3b6c4
Reviewed-on: https://review.typo3.org/48363


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>