Add note above the form fields about the use of the special page. The note/caution seems more appropriate to be on the special page itself and in the extension so third party users can benefit from it.

After that, delete this section on Meta-Wiki https://meta.wikimedia.org/wiki/Wikimedia_URL_Shortener#Delete_a_link. Majority of users will never have access to delete short urls so there's no point in putting this warning in a user help page.