Lint hand-written intra-rustdoc links

[clarfonthey]

Once rust-lang/rust#43466 lands, people will be able to write links directly to Rust items instead of hand-writing the links. We should consider at some point adding a lint that will look for links to items and lint to use the latter format instead.

Obviously, we'd want to wait before this is implemented in compiler land before linting, but it's good to add to the list.

[killercup]

rust-lang/rfcs#1946 is merged! The implementation is still quite some time away, though.

In the meantime, cf. #1007 (comment) for some more thought and links on documentation lints.

[sharksforarms]

Landed in 1.48: rust-lang/rust#43466

[jyn514]

Once rust-lang/rust#43466 lands, people will be able to write links directly to Rust items instead of hand-writing the links. We should consider at some point adding a lint that will look for links to items and lint to use the latter format instead.

See also the rustdoc issue for this: rust-lang/rust#75805