Fix xm man page statement about asynchonous operation

The man page for xm command states that many commands acts
asynchronously which is not true. According to my experience most
commands does act synchronously with several exceptions, like create,
shutdown, mem-set and vcpu-set commands so this fixes the paragraph
available in this xm man page.

Signed-off-by: Michal Novotny <minovotn@redhat.com>

docs/man/xm.pod.1

@@ -40,12 +40,10 @@ Most B<xm> commands require root privileges to run due to the
 communications channels used to talk to the hypervisor.  Running as
 non root will return an error.
 
-Most B<xm> commands act asynchronously, so just because the B<xm>
-command returned doesn't mean the action is complete.  This is
-important, as many operations on domains, like create and shutdown,
-can take considerable time (30 seconds or more) to bring the machine
-into a fully compliant state.  If you want to know when one of these
-actions has finished you must poll through B<xm list> periodically.
+Most B<xm> commands act synchronously, except maybe create, shutdown,
+mem-set and vcpu-set. The fact that the B<xm> command returned doesn't
+necessarily mean that the action is complete and you must poll through
+xm list periodically to detect that the operation completed.
 
 =head1 DOMAIN SUBCOMMANDS