[BUGFIX] Revert abusive deprecation

Due to obsolete phpDoc comments, BackendUtility::helpText()
was deprecated while it is still used in the Core.

Update phpDoc comments and remove deprecation.

Resolves: #47681
Releases: 6.0, 6.1, 6.2
Change-Id: I2ddc42b5c3082d22f031e2710614f749323da550
Reviewed-on: https://review.typo3.org/20296
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn