DOCS-15627 Noted SCRAM-SHA-256 as default user auth mechanism (#4011) (#4134)

* DOCS-15627 Noted SCRAM-SHA-256 as default user auth mechanism (#4011)

* Noted SCRAM-SHA-256 as default user auth mechanism

* Moved default notice

* Internal review feedback

* Checking rendering as note

* Wording clarification

* External review feedback

* Rendering fix

Author: nvillahermosa-mdb

source/reference/connection-string.txt

@@ -994,25 +994,25 @@ credentials are authenticated against the ``admin`` database.
 
    * - .. urioption:: authMechanism
 
-     - Specify the authentication mechanism that MongoDB will use to
-       authenticate the connection. Possible values include:
+       Default: SCRAM-SHA-256
+       
+     - Specify the authentication mechanism that MongoDB uses to
+       authenticate the connection. If you don't specify an
+       ``authMechanism`` but provide user credentials, MongoDB attempts
+       to use SCRAM-SHA-256. If this fails, it falls back to
+       SCRAM-SHA-1.
 
        - :ref:`SCRAM-SHA-1 <authentication-scram-sha-1>`
 
-       - :ref:`SCRAM-SHA-256 <authentication-scram-sha-256>` (*Added in MongoDB 4.0*)
+       - :ref:`SCRAM-SHA-256 <authentication-scram-sha-256>`
 
        - :ref:`MONGODB-X509 <security-auth-x509>`
 
        - ``MONGODB-AWS`` (*Added in MongoDB 4.4*)
 
        - :ref:`GSSAPI <security-auth-kerberos>` (Kerberos)
 
-       - :ref:`PLAIN <security-auth-ldap>` (LDAP SASL)
-
-       MongoDB 4.0 removes support for the ``MONGODB-CR``
-       authentication mechanism. You cannot specify ``MONGODB-CR`` as
-       the authentication mechanism when connecting to MongoDB 4.0+
-       deployments.
+       - :ref:`PLAIN <security-auth-ldap>` (LDAP SASL) 
 
        Only MongoDB Enterprise :binary:`~bin.mongod` and
        :binary:`~bin.mongos` instances provide ``GSSAPI`` (Kerberos) and