Document that groupByKey will OOM for large keys

This pull request is my own work and I license it under Spark's open-source license.

This contribution is an improvement to the documentation. I documented that the maximum number of values per key for groupByKey is limited by available RAM (see [Datablox][datablox link] and [the spark mailing list][list link]).

Just saying that better performance is available is not sufficient. Sometimes you need to do a group-by - your operation needs all the items available in order to complete. This warning explains the problem.

[datablox link]: http://databricks.gitbooks.io/databricks-spark-knowledge-base/content/best_practices/prefer_reducebykey_over_groupbykey.html
[list link]: http://apache-spark-user-list.1001560.n3.nabble.com/Understanding-RDD-GroupBy-OutOfMemory-Exceptions-tp11427p11466.html

Author: Eric Moyer <eric_moyer@yahoo.com>

Closes #3936 from RadixSeven/better-group-by-docs and squashes the following commits:

5b6f4e9 [Eric Moyer] groupByKey docs naming updates
238e81b [Eric Moyer] Doc that groupByKey will OOM for large keys