DOCSP-14875 clarify agg pipeline read pref runs on primary

Author: andf-mongodb

source/core/read-preference.txt

@@ -230,13 +230,14 @@ Additional Considerations
 -------------------------
 
 For :doc:`aggregation pipeline </core/aggregation-pipeline>`
-operations, you must run on the primary if the pipeline includes either
-the :pipeline:`$out` stage or the :pipeline:`$merge` stage.
+operations that include the :pipeline:`$out` or :pipeline:`$merge`
+stages, the pipeline runs on the :term:`primary` member regardless of
+read preference setting.
 
 For :dbcommand:`mapReduce` operations, only "inline"
 :dbcommand:`mapReduce` operations that do not write data support read
-preference. Otherwise, :dbcommand:`mapReduce` operations must run on
-the :term:`primary` members.
+preference. Otherwise, :dbcommand:`mapReduce` operations run on the
+:term:`primary` member.
 
 .. [#edge-cases-2-primaries]