Note potential additional API call to reload 'media_link'.

Addresses:
#1559 (comment)

Author: tseaver

gcloud/storage/blob.py

@@ -277,6 +277,11 @@ def delete(self, client=None):
     def download_to_file(self, file_obj, client=None):
         """Download the contents of this blob into a file-like object.
 
+        .. note::
+
+           If the server-set property, :attr:`media_link`, is not yet
+           initialized, makes an additional API request to load it.
+
         :type file_obj: file
         :param file_obj: A file handle to which to write the blob's data.