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