Merge pull request #505 from JoshRosen/SPARK-1026

Deprecate mapPartitionsWithSplit in PySpark (SPARK-1026)

This commit deprecates `mapPartitionsWithSplit` in PySpark (see [SPARK-1026](https://spark-project.atlassian.net/browse/SPARK-1026) and removes the remaining references to it from the docs.
(cherry picked from commit 05be7047744c88e64e7e6bd973f9bcfacd00da5f)

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