virsh: man: Document asynchronous behaviour of detach-device-alias

This command is fully async. Note that users can use virsh event to be
notified of the guest actually removing the device.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

tools/virsh.pod

@@ -3290,7 +3290,10 @@ I<--persistent>.
 =item B<detach-device-alias> I<domain> I<alias>
 [[[I<--live>] [I<--config>] | [I<--current>]]]]
 
-Detach a device with given I<alias> from the I<domain>.
+Detach a device with given I<alias> from the I<domain>. This command returns
+successfully after the unplug request was sent to the hypervisor. The actual
+removal of the device is notified asynchronously via libvirt events
+(see virsh event).
 
 If I<--live> is specified, affect a running domain.
 If I<--config> is specified, affect the next startup of a persistent domain.