Help message about save_file return added. (#138)

* Help message about save_file return added.

* Style changes

Author: AstraSerg

flask_pymongo/__init__.py

@@ -186,6 +186,7 @@ def get_upload(filename):
 
     def save_file(self, filename, fileobj, base="fs", content_type=None, **kwargs):
         """Save a file-like object to GridFS using the given filename.
+           Return the "_id" of the created file.
 
         .. code-block:: python