PYTHON-1615 Update Cursor.hint doc

Author: ajdavis

pymongo/cursor.py

@@ -827,9 +827,9 @@ def hint(self, index):
  Judicious use of hints can greatly improve query
  performance. When doing a query on multiple fields (at least
  one of which is indexed) pass the indexed field as a hint to
- the query. Hinting will not do anything if the corresponding
- index does not exist. Raises
- :class:`~pymongo.errors.InvalidOperation` if this cursor has
+ the query. Raises :class:`~pymongo.errors.OperationFailure` if the
+ provided hint requires an index that does not exist on this collection,
+ and raises :class:`~pymongo.errors.InvalidOperation` if this cursor has
  already been used.
 
  `index` should be an index as passed to