qmp-commands: move 'query-commands' doc to schema

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

qapi/common.json

@@ -114,6 +114,23 @@
 # Returns: A list of @CommandInfo for all supported commands
 #
 # Since: 0.14.0
+#
+# Example:
+#
+# -> { "execute": "query-commands" }
+# <- {
+#      "return":[
+#         {
+#            "name":"query-balloon"
+#         },
+#         {
+#            "name":"system_powerdown"
+#         }
+#      ]
+#    }
+#
+# Note: This example has been shortened as the real response is too long.
+#
 ##
 { 'command': 'query-commands', 'returns': ['CommandInfo'] }