DOCSP-33037 doc that some out stages are now filtered out (#614)

Author: kanchana-mongodb

source/query-plan.txt

@@ -23,6 +23,13 @@ On the :guilabel:`Explain Plan` modal, you can view the explain stages as a
 tree. You can also view the explain details in raw JSON format by selecting 
 the :guilabel:`Raw Output` view.
 
+.. note:: 
+
+   The :guilabel:`Explain Plan` doesn't show aggregation pipeline stages
+   such as :pipeline:`$merge` and :pipeline:`$out` because
+   |compass-short| ignores all out stages from the aggregation before 
+   running the explain plan.
+
 The explain plan includes a :guilabel:`Query Performance Summary` with 
 information on the execution of your query such as: