Add a note for context termination for History server on Yarn

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

docs/monitoring.md

@@ -173,6 +173,8 @@ follows:
 Note that in all of these UIs, the tables are sortable by clicking their headers,
 making it easy to identify slow tasks, data skew, etc.
 
+Note that the history server only displays completed Spark jobs. One way to signal the completion of a Spark job is to stop the Spark Context explicitly (`sc.stop()`), or in Python using the `with SparkContext() as sc:` to handle the Spark Context setup and tear down, and still show the job history on the UI.
+
 # Metrics
 
 Spark has a configurable metrics system based on the