Update docs to include labelSelectors functionality

[a-mccarthy]

Related to #4843.

Velero 1.9 adds the ability for backups and restores to filter resources on multiple labels using a new API spec field ,labelSelectors.

This is a separate feature from the existing resource filtering that allows users to select a single label to use to filter resources for a backup or restore.

more details in this design doc and implementation PR.

[a-mccarthy]

jk, these docs were already added. I missed that when looking the PR.