DOCSP-33135 What's New AWS Region Settings Link (#985)

* DOCSP-33135 What's New AWS Region Settings Link

* add ref link

* add aws webidentity link

* edit copy

* edit

* style edit

* rea review comments

(cherry picked from commit b56f50c)

Author: lindseymoore

source/whats-new.txt

@@ -69,14 +69,19 @@ The {+driver-short+} v6.1 release includes the following features:
   ``Decimal128.fromStringWithRounding()`` method. To learn more, see the
   `v6.1.0 bson release notes <https://github.com/mongodb/js-bson/releases/tag/v6.1.0>`__.
 
-- Detects environment variables for region settings when
-  authenticating by using the ``MONGODB-AWS`` authentication mechanism.
-  To instruct the driver to use your region options, you must set both
-  of the following environment variables:
-  
+- Detects environment variables for region settings when you
+  authenticate by using the `IAM AssumeRoleWithWebIdentity action
+  <https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRoleWithWebIdentity.html>`__ 
+  with ``MONGODB-AWS`` authentication. To instruct the driver to use your region options, 
+  you must set both of the following environment variables:
+
   - ``AWS_STS_REGIONAL_ENDPOINTS``
   - ``AWS_REGION``
 
+  To learn how to set region settings when using the ``MONGODB-AWS`` 
+  authentication mechanism, see the :guilabel:`Web Identity Token` tab in the 
+  :ref:`MONGODB-AWS <mongodb-aws>` section of the Authentication Mechanisms guide. 
+
 - Fixes a memory leak issue caused by recursive calls to the ``next()``
   method of the ``ChangeStream`` type.