Docs: Correct return type for `add_link()`, `edit_link()`, and `wp_update_link()`.

Follow-up to [8758], [15564].

Props marian1.
Fixes #64764.

git-svn-id: https://develop.svn.wordpress.org/trunk@62024 602fd350-edb4-49c9-b593-d223f7449a82