[ci skip] Make it clear what GD uses instead of resources

Closes phpGH-6342.

Author: b1rdex

UPGRADING

@@ -311,9 +311,10 @@ PHP 8.0 UPGRADE NOTES
     warning.
 
 - GD:
-  . The GD extension now uses objects as the underlying data structure for
-    images, rather than resources. These objects are completely opaque, i.e.
-    they don't have any methods.
+  . 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`.
   . The deprecated function image2wbmp() has been removed.
     RFC: https://wiki.php.net/rfc/image2wbmp
   . The deprecated functions png2wbmp() and jpeg2wbmp() have been removed.