SPARK-1109 wrong API docs for pyspark map function

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #73 from ScrapCodes/SPARK-1109/wrong-API-docs and squashes the following commits:

1a55b58 [Prashant Sharma] SPARK-1109 wrong API docs for pyspark map function

(cherry picked from commit 02836657cfec50bc6cc357541e40f8d36c90b352)
Signed-off-by: Matei Zaharia <matei@databricks.com>