As a follow up PR to #5544.

cc davies

Author: Reynold Xin <rxin@databricks.com>

Closes #5558 from rxin/sort-doc-improvement and squashes the following commits:

f4c276f [Reynold Xin] Review feedback.
d2dcf24 [Reynold Xin] Minor fix to SPARK-6958: Improve Python docstring for DataFrame.sort.