DOCSP-42956: Remove $collection support (#3138)

Adds a note about removed $collection and collection() support to the upgrade guide.

docs/upgrade.txt

@@ -109,6 +109,47 @@ This library version introduces the following breaking changes:
   of this behavior, you cannot have two separate ``id`` and ``_id`` fields in your
   documents.
 
+- Removes support for the ``$collection`` property. The following code shows
+  how to assign a MongoDB collection to a variable in your ``User`` class in
+  older versions compared to v5.0:
+
+  .. code-block:: php
+    :emphasize-lines: 10-11
+
+    use MongoDB\Laravel\Eloquent\Model;
+
+    class User extends Model
+    {
+        protected $keyType = 'string';
+
+        // older versions
+        protected $collection = 'app_user';
+
+        // v5.0
+        protected $table = 'app_user';
+
+        ...
+    }
+
+  This release also modifies the associated ``DB`` and ``Schema`` methods for
+  accessing a MongoDB collection. The following code shows how to access the
+  ``app_user`` collection in older versions compared to v5.0:
+
+  .. code-block:: php
+    :emphasize-lines: 9-11
+
+    use Illuminate\Support\Facades\Schema;
+    use Illuminate\Support\Facades\DB;
+    use MongoDB\Laravel\Schema\Blueprint;
+
+      // older versions
+      Schema::collection('app_user', function (Blueprint $collection) { ... });
+      DB::collection('app_user')->find($id);
+
+      // v5.0
+      Schema::table('app_user', function (Blueprint $table) { ... });
+      DB::table('app_user')->find($id);
+
 .. _laravel-breaking-changes-v4.x:
 
 Version 4.x Breaking Changes