Add better docs for coalesce.

Include the useful tip that if shuffle=true, coalesce can actually
increase the number of partitions.

This makes coalesce more like a generic `RDD.repartition` operation.

(Ideally this `RDD.repartition` could automatically choose either a coalesce or
a shuffle if numPartitions was either less than or greater than, respectively,
the current number of partitions.)