[SPARK-15660][CORE] Update RDD `variance/stdev` description and add popVariance/popStdev

## What changes were proposed in this pull request?

In Spark-11490, `variance/stdev` are redefined as the **sample** `variance/stdev` instead of population ones. This PR updates the other old documentations to prevent users from misunderstanding. This will update the following Scala/Java API docs.

- http://spark.apache.org/docs/2.0.0-preview/api/scala/index.html#org.apache.spark.api.java.JavaDoubleRDD
- http://spark.apache.org/docs/2.0.0-preview/api/scala/index.html#org.apache.spark.rdd.DoubleRDDFunctions
- http://spark.apache.org/docs/2.0.0-preview/api/scala/index.html#org.apache.spark.util.StatCounter
- http://spark.apache.org/docs/2.0.0-preview/api/java/org/apache/spark/api/java/JavaDoubleRDD.html
- http://spark.apache.org/docs/2.0.0-preview/api/java/org/apache/spark/rdd/DoubleRDDFunctions.html
- http://spark.apache.org/docs/2.0.0-preview/api/java/org/apache/spark/util/StatCounter.html

Also, this PR adds them `popVariance` and `popStdev` functions clearly.

## How was this patch tested?

Pass the updated Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13403 from dongjoon-hyun/SPARK-15660.