Add docs to storage.

Author: bojanz

src/OAuth2/Storage/RefreshTokenInterface.php

@@ -24,11 +24,11 @@ interface RefreshTokenInterface
      * @return
      * An associative array as below, and NULL if the refresh_token is
      * invalid:
-     * - refresh_token: Stored refresh token identifier.
-     * - client_id: Stored client identifier.
-     * - user_id: Stored user identifier.
-     * - expires: Stored expiration unix timestamp.
-     * - scope: (optional) Stored scope values in space-separated string.
+     * - refresh_token: Refresh token identifier.
+     * - client_id: Client identifier.
+     * - user_id: User identifier.
+     * - expires: Expiration unix timestamp, or 0 if the token doesn't expire.
+     * - scope: (optional) Scope values in space-separated string.
      *
      * @see http://tools.ietf.org/html/rfc6749#section-6
      *
@@ -54,7 +54,7 @@ public function getRefreshToken($refresh_token);
      * @param $user_id
      * User identifier to be stored.
      * @param $expires
-     * expires to be stored.
+     * Expiration timestamp to be stored. 0 if the token doesn't expire.
      * @param $scope
      * (optional) Scopes to be stored in space-separated string.
      *