[SPARK-2871] [PySpark] Add missing API

Try to bring all Java/Scala API to PySpark.
Same case here: this should be similarly descriptive to the Scala docs:

```scala
 /**
   * Zips this RDD with generated unique Long ids. Items in the kth partition will get ids k, n+k,
   * 2*n+k, ..., where n is the number of partitions. So there may exist gaps, but this method
   * won't trigger a spark job, which is different from [[org.apache.spark.rdd.RDD#zipWithIndex]].
   */
  def zipWithUniqueId(): RDD[(T, Long)] = {
```