Python: Add INTERNAL annotation to Response::InstanceSource

Since we need to reserve the flexibility to change this setup within the next
few months, we don't want to commit to keeping this extension point around for
the 12 months that the normal API deprecation cycle requires.

python/ql/src/semmle/python/frameworks/Flask.qll

@@ -99,6 +99,8 @@ module Flask {
     }
 
     /**
+     * INTERNAL: Do not use.
+     *
      * A source of instances of `flask.Response`, extend this class to model new instances.
      *
      * This can include instantiations of the class, return values from function