Update batch_size docstring PYTHON-350

Author: behackett

pymongo/cursor.py

@@ -307,7 +307,14 @@ def limit(self, limit):
  return self
 
  def batch_size(self, batch_size):
- """Set the size for batches of results returned by this cursor.
+ """Limits the number of documents returned in one batch. Each batch
+ requires a round trip to the server. It can be adjusted to optimize
+ performance and limit data transfer.
+
+ .. note:: batch_size can not override MongoDB's internal limits on the
+ amount of data it will return to the client in a single batch (i.e
+ if you set batch size to 1,000,000,000, MongoDB will currently only
+ return 4-16MB of results per batch).
 
  Raises :class:`TypeError` if `batch_size` is not an instance
  of :class:`int`. Raises :class:`ValueError` if `batch_size` is