Fixes #2195 by moving the suggestion into a user hint and adding a sentence about possible impacts on relocatable sites.

New error looks like:

Issues found in 1 input. Find details below.

[a.md]:
 [ERROR] error: (at 4:1) | Cannot resolve root-relative link '/a'

🔍 3 Total (in 60ms) 🔗 3 Unique ✅ 0 OK 🚫 3 Errors

Hint: Unresolved root-relative links in local files. To resolve root-relative links to a local directory, provide a root dir. Alternatively, if the site should be relocatable, consider using locally-relative links.

Happy to tweak any part of the phrasing.

I've kept it as "locally-relative" (rather than @ roberth's suggestion of page-/document-relative), because I think it's more straightforward when it refers to files on disk. Page and document both need the user to think about web pages before connecting it a file path. MDN calls them current directory relative. That's a bit long but I also wouldn't mind "directory-relative".