Update module_loading documents

Author: blakegong

graphene/utils/module_loading.py

@@ -5,7 +5,10 @@
 def import_string(dotted_path, dotted_attributes=None):
     """
     Import a dotted module path and return the attribute/class designated by the
-    last name in the path. Raise ImportError if the import failed.
+    last name in the path. When a dotted attribute path is also provided, the
+    dotted attribute path would be applied to the attribute/class retrieved from
+    the first step, and return the corresponding value designated by the
+    attribute path. Raise ImportError if the import failed.
     """
     try:
         module_path, class_name = dotted_path.rsplit('.', 1)