[FEATURE] Enhance templateRootPath handling and documentation

Releases: 6.2
Resolves: #62645
Change-Id: I06343c99d41253a56ea2f77fdc5d833da3ba5892
Reviewed-on: http://review.typo3.org/34477
Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>