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

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 2.1.0
• Component/s: Spark Core
• Assignee: Dongjoon Hyun
• Reporter: Dongjoon Hyun
• Created: 31/May/16 05:21
• Updated: 23/Jun/16 10:08
• Resolved: 23/Jun/16 10:07

Description

In Spark-11490, `variance/stdev` are redefined as the sample variance/stdev instead of population ones. This PR updates the comments to prevent users from misunderstanding. This will update the following 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

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

Issue Links

• relates to: SPARK-11490 variance should alias var_samp instead of var_pop
• links to: [Github] Pull Request #13403 (dongjoon-hyun)