[SPARK-2013] Documentation for saveAsPickleFile and pickleFile in Python

Author: Kan Zhang <kzhang@apache.org>

Closes #983 from kanzhang/SPARK-2013 and squashes the following commits:

0e128bb [Kan Zhang] [SPARK-2013] minor update
e728516 [Kan Zhang] [SPARK-2013] Documentation for saveAsPickleFile and pickleFile in Python

(cherry picked from commit b52603b039cdfa0f8e58ef3c6229d79e732ffc58)
Signed-off-by: Reynold Xin <rxin@apache.org>

Conflicts:
	docs/programming-guide.md