[SPARK-2871] [PySpark] Add missing API

Try to bring all Java/Scala API to PySpark.
The Scala documentation is much more descriptive about what this method does:

```scala
  /**
   * Zips this RDD with its element indices. The ordering is first based on the partition index
   * and then the ordering of items within each partition. So the first item in the first
   * partition gets index 0, and the last item in the last partition receives the largest index.
   * This is similar to Scala's zipWithIndex but it uses Long instead of Int as the index type.
   * This method needs to trigger a spark job when this RDD contains more than one partitions.
   */
  def zipWithIndex(): RDD[(T, Long)] = new ZippedWithIndexRDD(this)
```

The Python documentation should explain these subtleties, too.