Intra-doc links should implicitly add Self:: prefix as needed

[clarfonthey]

It's extremely common to reference methods on the same type when writing docs. For example, Vec::reserve_exact mentions Vec::reserve.

In almost all of these cases, the method being linked doesn't include the full type or Self:: in the actual text, and has to resort to copying the method name with Self:: in the link destination.

It would be nice if rustdoc automatically resolved these links without the need to add the extra Self:: prefix. For example, writing [`reserve`] should automatically resolve to Self::reserve if a reserve item doesn't exist on its own. If both the method and the standalone item exist, we could add a lint to point out the ambiguity, requiring the user to clarify either self::reserve or Self::reserve; this feels like the best move to avoid changing the existing behaviour.

Either way, this pattern feels very common and seems like a papercut that makes documentation just a little bit harder to write.

[jyn514]

Duplicate of #59039