Remove return descriptions from the DocBlocks for `add_image_size()` and `set_post_thumbnail_size()` as they don't return anything.

Also document the `$_wp_additional_image_sizes` global, leveraged in `add_image_size()`.

Props stevegrunwell.
Fixes #30511.


git-svn-id: https://develop.svn.wordpress.org/trunk@30633 602fd350-edb4-49c9-b593-d223f7449a82