qmp-commands: move 'migrate_cancel' 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

@@ -270,18 +270,6 @@ Notes:
 (3) The user Monitor's "detach" argument is invalid in QMP and should not
     be used
 
-migrate_cancel
---------------
-
-Cancel the current migration.
-
-Arguments: None.
-
-Example:
-
--> { "execute": "migrate_cancel" }
-<- { "return": {} }
-
 migrate-incoming
 ----------------
 

qapi-schema.json

@@ -2587,6 +2587,12 @@
 # Notes: This command succeeds even if there is no migration process running.
 #
 # Since: 0.14.0
+#
+# Example:
+#
+# -> { "execute": "migrate_cancel" }
+# <- { "return": {} }
+#
 ##
 { 'command': 'migrate_cancel' }