document background indexing option

Author: kchodorow

pymongo/collection.py

@@ -677,6 +677,8 @@ def ensure_index(self, key_or_list, deprecated_unique=None,
  - `unique`: should this index guarantee uniqueness?
  - `dropDups` or `drop_dups`: should we drop duplicates
  during index creation when creating a unique index?
+ - `background`: if this index should be created in the
+ background
  - `min`: minimum value for keys in a :data:`~pymongo.GEO2D`
  index
  - `max`: maximum value for keys in a :data:`~pymongo.GEO2D`