DOCS-16003: explain in aggregate is unstable when using stable api (#3729)

* DOCS-16003: explain in aggregate is unstable when using stable api

* revise content

* JW comments

Author: rustagir

source/reference/command/aggregate.txt

@@ -317,6 +317,9 @@ When using :ref:`Stable API <stable-api>` V1:
   - :pipeline:`$planCacheStats`
   - :pipeline:`$search`
 
+- Don't include the ``explain`` field in an :dbcommand:`aggregate`
+  command. If you do, the server returns an :ref:`APIStrictError <api-strict-resp>` error.
+
 - When using the :pipeline:`$collStats` stage, you can only use the
   ``count`` field. No other :pipeline:`$collStats` fields are
   available.