Improve inline documentation for the `multi_resize()` method in `WP_Image_Editor`, `WP_Image_Editor_GD`, and `WP_Image_Editor_Imagick`.

Props paolal for the initial patch.
Fixes #25530.


git-svn-id: https://develop.svn.wordpress.org/trunk@25728 602fd350-edb4-49c9-b593-d223f7449a82