DOCS-16217 Add Authsource Behavior to MongoDump Page (#94)

* DOCS-16217 Add Authsource Behavior to MongoDump Page

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* Add example

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Author: ianf-mongodb

source/mongodump.txt

@@ -264,6 +264,15 @@ Behavior
    the :dbtools:`--archive </mongodump/#std-option-mongodump.--archive>` 
    option.  
 
+Using ``mongodump`` Without an ``authSource``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When :urioption:`authSource` is not specified in the MongoDB URI, the 
+database name specified in :option:`--db` is used both to authenticate 
+your ``mongodump`` session and to indicate the database being dumped. For 
+an example of using a different database for authentication when using 
+``mongodump``, see :ref:`mongodump-auth-dump`.
+
 Restore to Matching Server Version
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1098,3 +1107,22 @@ connect to a MongoDB Atlas cluster:
 .. code-block:: none
 
    mongodump 'mongodb+srv://cluster0.example.com/testdb?authSource=$external&authMechanism=MONGODB-AWS'  <other options>
+
+.. _mongodump-auth-dump:
+
+Authenticating with a Specific Database
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To authenticate with a different database than the one being dumped, you 
+must specify ``authSource`` in the MongoDB URI.
+
+In this example:
+
+- The username ``myuser`` and password ``mypassword`` is used. This user
+  has read access to ``testdb``.
+- The ``admin`` database is used to authenticate the user.
+- The ``testdb`` database is being dumped.
+
+.. code-block:: none
+
+   mongodump 'mongodb+srv://myuser:[email protected]/?authSource=admin' --db testdb