[BUGFIX] Fix wrong comment in TSFE for xhtmlDoctype property

The comment was wrong because the TSFE-Controller
takes care of setting this option internally.

Resolves: #81219
Releases: master, 8.7
Change-Id: If4af5b5aefa2758063e595e9e3e0b55892c7df27
Reviewed-on: https://review.typo3.org/52851


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>