doc: document that Domain-0 can't be migrated across cpupools

Domain-0 is always member of Pool-0 (or, to be precise: of the cpuppol
with cpupool-id 0). Document this in the xl man page.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

docs/man/xl.pod.1

@@ -1263,6 +1263,7 @@ exact same syntax as in B<cpupool-cpu-add> above.
 =item B<cpupool-migrate> I<domain> I<cpu-pool>
 
 Moves a domain specified by domain-id or domain-name into a cpu-pool.
+Domain-0 can't be moved to another cpu-pool.
 
 =item B<cpupool-numa-split>