SPARK-1344 [DOCS] Scala API docs for top methods

Use "k" in javadoc of top and takeOrdered to avoid confusion with type K in pair RDDs. I think this resolves the discussion in SPARK-1344.

Author: Sean Owen <sowen@cloudera.com>

Closes #3168 from srowen/SPARK-1344 and squashes the following commits:

6963fcc [Sean Owen] Use "k" in javadoc of top and takeOrdered to avoid confusion with type K in pair RDDs