DOCS-16285-slotBasedPlan-internal-only-note-v6.0-backport (#5774)

Author: mmaville-mdb

source/reference/explain-results.txt

@@ -363,7 +363,7 @@ representative. Your output may differ significantly.
    .. data:: explain.queryPlanner.winningPlan.slotBasedPlan
 
       Document with information about the slot based query execution plan
-      tree and stages.
+      tree and stages. **For internal use by MongoDB.**
 
       .. versionadded:: 5.1