[ci skip] Add note that imagedestroy() no longer has an effect

Closes GH-6427.

Author: xPaw

UPGRADING

@@ -314,7 +314,9 @@ PHP 8.0 UPGRADE NOTES
   . The GD extension now uses a GdImage objects as the underlying data structure
     for images, rather than resources. These objects are completely opaque, i.e.
     they don't have any methods. Return value checks using is_resource() should
-    be replaced with checks for `false`.
+    be replaced with checks for `false`. The imagedestroy() function no longer
+    has an effect, instead the GdImage instance is automatically destroyed if
+    it is no longer referenced.
   . The deprecated function image2wbmp() has been removed.
     RFC: https://wiki.php.net/rfc/image2wbmp
   . The deprecated functions png2wbmp() and jpeg2wbmp() have been removed.