Fix docblock to what its interface contracts says.

src/ORM/Table.php

@@ -1880,7 +1880,7 @@ public function exists($conditions)
      *
      * @param \Cake\Datasource\EntityInterface $entity
      * @param array $options
-     * @return bool|\Cake\Datasource\EntityInterface|mixed
+     * @return \Cake\Datasource\EntityInterface|false
      * @throws \Cake\ORM\Exception\RolledbackTransactionException If the transaction is aborted in the afterSave event.
      */
     public function save(EntityInterface $entity, $options = [])