Warn about path change for #[doc(include)]

[hdevalence]

Recently (#60938) the path root for #[doc(include)] files was changed. This change makes a lot of sense, and it's not difficult to update, but there is now a situation where code using the old path roots won't compile on new nightlies and vice versa.

Because the error message is just that the file is missing, there's a lot of confusion (e.g., dalek-cryptography/bulletproofs#290, dalek-cryptography/bulletproofs#282, dalek-cryptography/bulletproofs#292, dalek-cryptography/curve25519-dalek#269, dalek-cryptography/curve25519-dalek#275) about build failures.

Would it be possible to add a warning to the "file missing" error for #[doc(include)] noting that the path changed recently? This won't help the case of compiling new-doc-paths-on-old-nightlies but might give a hint about what's going on when there's a build failure with old-doc-paths-on-new-nightlies.

[jyn514]

This changed over 15 months ago and I plan to deprecate doc(include) on the next beta bump (in favor of #78835), so I don't think it makes sense to add at this point. I'm sorry that this change was not communicated clearly at the time.