virsh: Clarify documentation for virsh dompmsuspend command

Clarify the docs to make more clear what this command does and that it
requires a guest agent running in the guest.

tools/virsh.c

@@ -2765,8 +2765,11 @@ cmdSuspend(vshControl *ctl, const vshCmd *cmd)
  * "dompmsuspend" command
  */
 static const vshCmdInfo info_dom_pm_suspend[] = {
-    {"help", N_("suspend a domain for a given time duration")},
-    {"desc", N_("Suspend a running domain for a given time duration.")},
+    {"help", N_("suspend a domain gracefully using power management "
+                "functions")},
+    {"desc", N_("Suspends a running domain using guest OS's power management. "
+                "(Note: This requires a guest agent configured and running in "
+                "the guest OS).")},
     {NULL, NULL}
 };
 

tools/virsh.pod

@@ -1493,7 +1493,7 @@ Moves a domain out of the suspended state.  This will allow a previously
 suspended domain to now be eligible for scheduling by the underlying
 hypervisor.
 
-=item B<dompmsuspend> I<domain-id> I<target>
+=item B<dompmsuspend> I<domain-id> I<target> [I<--duration>]
 
 Suspend a running domain into one of these states (possible I<target>
 values):
@@ -1501,6 +1501,14 @@ values):
     disk equivallent of S4 ACPI state
     hybrid RAM is saved to disk but not powered off
 
+The I<--duration> argument specifies number of seconds before the domain is
+woken up after it was suspended (see also B<dompmwakeup>). Default is 0 for
+unlimited suspend time. (This feature isn't currently supported by any
+hypervisor driver and 0 should be used.).
+
+Note that this command requires a guest agent configured and running in the
+domain's guest OS.
+
 =item B<dompmwakeup> I<domain-id>
 
 Wakeup a domain suspended by dompmsuspend command. Injects a wakeup