qmp-commands: move 'system_powerdown' doc to schema

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/qmp-commands.txt

@@ -137,18 +137,6 @@ Example:
 -> { "execute": "system_wakeup" }
 <- { "return": {} }
 
-system_powerdown
-----------------
-
-Send system power down event.
-
-Arguments: None.
-
-Example:
-
--> { "execute": "system_powerdown" }
-<- { "return": {} }
-
 device_add
 ----------
 

qapi-schema.json

@@ -2174,6 +2174,11 @@
 #        returning does not indicate that a guest has accepted the request or
 #        that it has shut down.  Many guests will respond to this command by
 #        prompting the user in some way.
+# Example:
+#
+# -> { "execute": "system_powerdown" }
+# <- { "return": {} }
+#
 ##
 { 'command': 'system_powerdown' }