xfs_scrub: warn about difficult rtgroup repairs

Codex noticed that rtgroup metadata repairs can be difficult (e.g.
rtrmap) but we don't ever warn the user about that.  Scrub does that for
per-AG and fs metadata, so do that here too.

Cc: <linux-xfs@vger.kernel.org> # v6.13.0
Fixes: 241d915d69d4ae ("xfs_scrub: scrub realtime allocation group metadata")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>