[HttpFoundation] Document MongoDbSessionHandler changes

UPGRADE-2.2.md

@@ -1,6 +1,15 @@
 UPGRADE FROM 2.1 to 2.2
 =======================
 
+### HttpFoundation
+
+ * The MongoDbSessionHandler default field names and timestamp type have changed.
+
+   The `sess_` prefix was removed from default field names. The session ID is
+   now stored in the `_id` field by default. The session date is now stored as a
+   `MongoDate` instead of `MongoTimestamp`, which also makes it possible to use
+   TTL collections in MongoDB 2.2+ instead of relying on the `gc()` method.
+
 #### Deprecations
 
  * The `Request::splitHttpAcceptHeader()` is deprecated and will be removed in 2.3.

src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php

@@ -36,6 +36,13 @@ class MongoDbSessionHandler implements \SessionHandlerInterface
     /**
      * Constructor.
      *
+     * List of available options:
+     *  * database: The name of the database [required]
+     *  * collection: The name of the collection [required]
+     *  * id_field: The field name for storing the session id [default: _id]
+     *  * data_field: The field name for storing the session data [default: data]
+     *  * time_field: The field name for storing the timestamp [default: time]
+     *
      * @param \Mongo $mongo   A "Mongo" instance
      * @param array  $options An associative array of field options
      *