DOCSP-36792: Add default database connection admonition (#2888)

Adds an admonition that advises users to set their default database connection to MongoDB

docs/query-builder.txt

@@ -46,6 +46,19 @@ The following example shows the syntax of a query builder call:
    DB::collection('<collection name>')
        // chain methods by using the "->" object operator
        ->get();
+.. tip::
+
+   Before using the ``DB::collection()`` method, ensure that you specify MongoDB as your application's
+   default database connection. For instructions on setting the database connection,
+   see the :ref:`laravel-quick-start-connect-to-mongodb` step in the Quick Start.
+
+   If MongoDB is not your application's default database, you can use the ``DB::connection()`` method
+   to specify a MongoDB connection. Pass the name of the connection to the ``connection()`` method,
+   as shown in the following code:
+
+   .. code-block:: php
+
+      $connection = DB::connection('mongodb');
 
 This guide provides examples of the following types of query builder operations: