[CLEANUP] Improve comments in TypoScriptFrontendController

Resolves: #57814
Releases: 6.2
Change-Id: I0f92a0695f5897eb0311f68bfd21ebb40d9303d8
Reviewed-on: https://review.typo3.org/29355
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Oliver Klee
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel