Rook Velero Backup document does not warn against the opt-out feature

[surajssd]

Rook Velero document, does not caution about the restic parameter require_volume_annotation which is set to false by default hence all the volumes are backed up without the need of the annotation.

Update the document to set this parameter to require_volume_annotation = true explicitly and in the annotation part mention that if you want to reduce the hassle of annotating the pods then set the parameter require_volume_annotation to false or leave it altogether.