qmp-commands: move 'cpu-add' doc to schema

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

qapi-schema.json

@@ -2203,6 +2203,12 @@
 # Returns: Nothing on success
 #
 # Since: 1.5
+#
+# Example:
+#
+# -> { "execute": "cpu-add", "arguments": { "id": 2 } }
+# <- { "return": {} }
+#
 ##
 { 'command': 'cpu-add', 'data': {'id': 'int'} }