docs: comments wiping supported algorithms

The current scrub version doesn't support pfitzner7, pfitzner33 and schneier
patterns on RHEL, we should comment it in virsh man page.

* tools/virsh.pod: update wiping algorithms docs.
Signed-off-by: NAlex Jia <ajia@redhat.com>

tools/virsh.pod

@@ -2081,6 +2081,9 @@ B<Supported algorithms>
   pfitzner33 - Roy Pfitzner's 33-random-pass method: random x33.
   random     - 1-pass pattern: random.
 
+B<Note>: The availability of algorithms may be limited by the version
+of the C<scrub> binary installed on the host.
+
 =item B<vol-dumpxml> [I<--pool> I<pool-or-uuid>] I<vol-name-or-key-or-path>
 
 Output the volume information as an XML dump to stdout.