Deprecate mapPartitionsWithSplit in PySpark.

Also, replace the last reference to it in the docs.

This fixes SPARK-1026.
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.