[SPARK-2108] Mark SparkContext methods that return block information as developer API's

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

Closes #1047 from ScrapCodes/SPARK-2108/mark-as-dev-api and squashes the following commits:

073ee34 [Prashant Sharma] [SPARK-2108] Mark SparkContext methods that return block information as developer API's
(cherry picked from commit e508f599f88baaa31a3498fb0bdbafdbc303119e)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>