Add xfs_scrub_fail systemd service file docs entry

Signed-off-by: Nathan Scott <nathans@debian.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>

diff --git a/scrub/xfs_scrub_fail@.service.in b/scrub/xfs_scrub_fail@.service.in
index 785f881..8d106e9 100644
--- a/scrub/xfs_scrub_fail@.service.in
+++ b/scrub/xfs_scrub_fail@.service.in
@@ -1,5 +1,6 @@
 [Unit]
 Description=Online XFS Metadata Check Failure Reporting for %I
+Documentation=man:xfs_scrub(8)
 
 [Service]
 Type=oneshot