[TASK] Improve descriptions of the rootline related methods

To highlight difference between BackendUtility::BEgetRootLine()
and RootlineUtility->get()

Resolves: #91455
Releases: 9.5, master
Change-Id: I63d7ca395d5a052d29d718316474b69d6519ebc9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64554


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>