Docs update to clarify Gitaly Cluster and snapshots in alternative backup has limitations

Problem to solve

In out backup documentation at https://docs.gitlab.com/ee/raketasks/backup_restore.html#alternative-backup-strategies we suggest

snapshots as part of your backup strategy

when in Gitaly Cluster https://docs.gitlab.com/ee/administration/gitaly/#snapshot-backup-and-recovery-limitations we specify

Gitaly Cluster does not support snapshot backups

This update is to clarify this limitation in the backup documentation.

Further details

Proposal

Who can address the issue

Other links/references