Add warning comment so that folks don't change this

Author: damccorm

sdks/python/apache_beam/yaml/yaml_provider.py

@@ -661,6 +661,9 @@ def create_transform(self, type, args, yaml_create_transform):
     return self._transform_factories[type](yaml_create_transform, **args)
 
 
+# Note: This function is used to override the default provider by some
+# users, so a change here will be breaking to those users. Change with
+# caution.
 def get_default_sql_provider():
   return beam_jar(
       urns={'Sql': 'beam:external:java:sql:v1'},