Request: Add note to URL Redirects about the requirement for dead links

[TacoLlama]

Please add a note on either the /redirects page or /redirects/new page that redirects only work on dead pages. I think the functionality is fine, and makes sense. But the current description lead me to think it would work on live pages and it was just taking time to propagate.

Current description: "Usually, this is an old page the visitors can no longer access because the content was moved to a different URL or deleted."

Requested description: "This is a page that visitors cannot access, usually because it's an old page that was moved to a different URL or deleted."