[DOC] Clarify phpdoc of SimpleFileBackend that lifetime is unsupported

Resolves: #78579
Releases: master, 7.6
Change-Id: I9260256630a6916bcc5b8cb8b93041f0a3bcf444
Reviewed-on: https://review.typo3.org/50508


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>