[TASK] Improve linkvalidator scheduler task usablity

The getAdditionalInformation() method is implemented in the
linkvalidator scheduler task to show more information about
the task in the scheduler Backend module.

Resolves: #61198
Releases: 6.3
Change-Id: Iebb43923b73ff4ecebab4a6747655596be859604
Reviewed-on: http://review.typo3.org/32383
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>