[BUGFIX] @return for TYPO3\CMS\Sv\AuthenticationService::authUser

Adjust the @return documentation of authUser() to match the actual
implementation in
TYPO3\CMS\Core\Authentication\AbstractUserAuthentication

Change-Id: I2d94cdfee6c58de80c7ec2be2b644b5fcd6c9a97
Resolves: #56421
Releases: 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/27943
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel