SPARK-2146.  Fix takeOrdered doc

Removes Python syntax in Scaladoc, corrects result in Scaladoc, and removes irrelevant cache() call in Python doc.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1086 from sryza/sandy-spark-2146 and squashes the following commits:

185ff18 [Sandy Ryza] Use Seq instead of Array
c996120 [Sandy Ryza] SPARK-2146.  Fix takeOrdered doc