[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

P.S. Accidentally closed #2123. New commits didn't show up after I reopened the PR. I've opened this instead and closed the old one.
Maybe we should mention `sampleByKey` first and let users know this doesn't give the exact sample size.