Documentation: Encourage use of reduceByKey instead of groupByKey.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #784 from pwendell/group-by-key and squashes the following commits:

9b4505f [Patrick Wendell] Small fix
6347924 [Patrick Wendell] Documentation: Encourage use of reduceByKey instead of groupByKey.
(cherry picked from commit 21570b463388194877003318317aafd842800cac)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>