[TASK] Mark various functionality as internal within TSFE

Use @internal where access private was marked, also add
some meaning to currently public methods that should only
be used by TYPO3 Core.

Resolves: #86442
Releases: master
Change-Id: If90f1a6dc6cad1edb0db2179cb87f5e6eb18f2ec
Reviewed-on: https://review.typo3.org/58459


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>