tweak: add ref to new defaults page

Author: kay-kim

source/core/transactions.txt

@@ -523,8 +523,10 @@ You can set the transaction-level :doc:`read concern
 - If transaction-level and the session-level read concern are unset,
   the transaction-level read concern defaults to the client-level read
   concern. By default, client-level read concern is
-  :readconcern:`"local"` for reads against the primary. See also
-  :ref:`transactions-read-preference`.
+  :readconcern:`"local"` for reads against the primary. See also:
+
+  - :ref:`transactions-read-preference`
+  - :doc:`/reference/mongodb-defaults`
 
 Transactions support the following read concern levels:
 
@@ -608,8 +610,10 @@ You can set the transaction-level :doc:`write concern
 
 - 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\>>`.
+  client-level write concern. By default, client-level write concern is
+  :writeconcern:`w: 1 <\<number\>>`. See also
+  :doc:`/reference/mongodb-defaults`.
+  
 
 Transactions support all write concern :ref:`w <wc-w>`
 values, including: