DOCSP-13570 Add docs on how to use explain (#107)

Author: kanchana-mongodb

source/supported-unsupported/mql-support.txt

@@ -215,13 +215,25 @@ Diagnostic Commands
        The following ``verbosity`` modes are supported: 
 
        - ``queryPlanner`` - provides information on the query plan.
-       - ``queryPlannerExtended`` - provides detailed information on the 
-         query plan including information about the |s3| objects, such as 
-         the |s3| object names and sizes, that will be queried.
+       - ``queryPlannerExtended`` - provides detailed information on 
+         the query plan including information about the |s3| objects, 
+         such as the |s3| object names and sizes, that will be queried.
 
        The ``executionStats`` and ``allPlansExecution`` modes are not 
        supported.
 
+       .. example:: 
+
+          The following example shows how to use the ``explain`` 
+          command to get information about the :manual:`aggregate 
+          </reference/command/aggregate>` command, including detailed 
+          information on the query plan.
+          
+          .. code-block:: sh 
+             :copyable: false 
+
+             db.runCommand({ "explain": { "aggregate": "user", "verbosity": "queryPlannerExtended", "pipeline": [ ], "cursor": {} }})
+
    * - :manual:`getLog </reference/command/getlog>`
 
      - The response returns successfully, but includes no log data.