DOCS-14791 default write concern (mongodb#6079)

Co-authored-by: jason-price-mongodb <jshfjghsdfgjsdjh@aolsdjfhkjsdhfkjsdf.com>

Author: jason-price-mongodb

source/core/transactions.txt

@@ -643,11 +643,19 @@ You can set the transaction-level :doc:`write concern
   concern for the commit.
 
 - If the transaction-level write concern and the session-level write
-  concern are unset, transaction-level write concern defaults to the
-  client-level write concern. By default, client-level write concern is
-  :writeconcern:`w: 1 <\<number\>>`. See also
-  :doc:`/reference/mongodb-defaults`.
-  
+  concern are unset, the transaction-level write concern defaults to the
+  client-level write concern of:
+
+  - :writeconcern:`w: "majority" <"majority">` in MongoDB 5.0 and later,
+    with differences for deployments containing :ref:`arbiters
+    <replica-set-arbiter-configuration>`. See
+    :ref:`wc-default-behavior`.
+
+  - :writeconcern:`w: 1 <\<number\>>` in MongoDB 4.4 and earlier.
+
+.. seealso::
+
+   :doc:`/reference/mongodb-defaults`
 
 Transactions support all write concern :ref:`w <wc-w>`
 values, including: