Update transaction lock docs

lib/transaction.js

@@ -61,8 +61,25 @@ Transaction.ISOLATION_LEVELS = {
  *   lock: t1.LOCK.SHARE,
  *   lock: t1.LOCK.KEY_SHARE, // Postgres 9.3+ only
  *   lock: t1.LOCK.NO_KEY_UPDATE // Postgres 9.3+ only
- * })
+ * });
  * ```
+ * 
+ * Postgres also supports specific locks while eager loading by using OF.
+ * 
+ * ```js
+ * UserModel.findAll({
+ *   where: ...,
+ *   include: [TaskModel, ...]
+ * }, {
+ *   transaction: t1,
+ *   lock: {
+ *     level: t1.LOCK...,
+ *     of: UserModel
+ *   }
+ * });
+ * ```
+ * UserModel will be locked but TaskModel won't!
+ * 
  * @property LOCK
  */
 Transaction.LOCK = Transaction.prototype.LOCK = {