DOCS-15627 Noted SCRAM-SHA-256 as default user auth mechanism (#274)

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

* Rebuild

* Rebuild

* Clarified SCRAM-SHA-256 vs SCRAM-SHA-1 defaults

* Clarified SCRAM-SHA-256 vs SCRAM-SHA-1 defaults

* Forgot a colon

* Clarified SCRAM-SHA-256 vs SCRAM-SHA-1 defaults

* Removed note styling

* External review feedback

Author: nvillahermosa-mdb

source/reference/options.txt

@@ -386,20 +386,11 @@ Authentication Options
 
 .. option:: --authenticationMechanism <name>
 
-   *Default*: SCRAM-SHA-1
-
    Specifies the authentication mechanism the |mdb-shell| uses to
-   authenticate to the :binary:`~bin.mongod` or :binary:`~bin.mongos`.
-
-   .. note::
-
-      Starting in version 4.0:
-
-      - MongoDB removes support for the deprecated MongoDB
-        Challenge-Response (``MONGODB-CR``) authentication mechanism.
-
-      -  MongoDB adds support for SCRAM mechanism using the SHA-256 hash
-         function (``SCRAM-SHA-256``).
+   authenticate to the :binary:`~bin.mongod` or :binary:`~bin.mongos`. 
+   If you don't specify an ``authenticationMechanism`` but provide user
+   credentials, the MongoDB Shell and drivers attempt to use
+   SCRAM-SHA-256. If this fails, they fall back to SCRAM-SHA-1.
 
    .. list-table::
       :header-rows: 1