PYTHON-1847 - Update countDocuments docstring

Author: behackett

pymongo/collection.py

@@ -1633,6 +1633,9 @@ def estimated_document_count(self, **kwargs):
  def count_documents(self, filter, session=None, **kwargs):
  """Count the number of documents in this collection.
 
+ .. note:: For a fast count of the total documents in a collection see
+ :meth:`estimated_document_count`.
+
  The :meth:`count_documents` method is supported in a transaction.
 
  All optional parameters should be passed as keyword arguments