more documentation PYTHON-168

Author: RedBeard0531

gridfs/__init__.py

@@ -196,6 +196,9 @@ def delete(self, file_id):
  file. Care should be taken to avoid concurrent reads to a file
  while it is being deleted.
 
+ .. note:: Deletes of non-existent files are considered successful
+ since the end result is the same: no file with that _id remains.
+
  :Parameters:
  - `file_id`: ``"_id"`` of the file to delete