[SPARK-2839][MLlib] Stats Toolkit documentation updated

Documentation updated for the Statistics Toolkit of MLlib. @mengxr @atalwalkar

https://issues.apache.org/jira/browse/SPARK-2839
`PairRDDFunctions` is in Scala only. We should avoid it here. Also, `sampleByKeyExact` is not available in Python. It may be useful to describe what keys and values mean in an RDD of key-value pairs in stratified sampling.