The history server on Yarn only shows completed jobs. This adds a note concerning the needed explicit context termination at the end of a spark job which is a best practice anyway.
Related to SPARK-2972 and SPARK-3458

Author: moussa taifi <moutai10@gmail.com>

Closes #4721 from moutai/add-history-server-note-for-closing-the-spark-context and squashes the following commits:

9f5b6c3 [moussa taifi] Fix upper case typo for YARN
3ad3db4 [moussa taifi] Add context termination for History server on Yarn