Add deprecation warning to _unwrap()

src/Collection/CollectionTrait.php

@@ -725,11 +725,12 @@ trait CollectionTrait
      * Backwards compatible wrapper for unwrap()
      *
      * @return \Traversable
-     * @deprecated
+     * @deprecated 3.0.10 Will be removed in 4.0.0
      */
     // @codingStandardsIgnoreLine
     public function _unwrap()
     {
+        deprecationWarning('CollectionTrait::_unwrap() is deprecated. Use CollectionTrait::unwrap() instead.');
         return $this->unwrap();
     }