Add docs for intra-doc-links

[Manishearth]

Fixes #66000

[rust-highfive]

r? @GuillaumeGomez

(rust_highfive has picked a reviewer for you, use r? to override)

[jyn514]

This should mention which scope is used to resolve the links: the parent module of the item. In the case of macros this is the crate root; in the case of re-exports it is the original module; in the case of impls I believe it's the parent of the type (but not sure).

[Manishearth]

@jyn514 isn't it just the def site scope? I don't think there's any special scoping here

[jyn514]

Sure, it's obvious to us because we implemented it, but it's not obvious to everyone else. See e.g. #72243 which is a bug report that macros aren't resolved using lexical scope instead of resolution scope.

[Manishearth]

@jyn514 Oh, I do consider that special scoping, I wasn't aware of that. For everything else it's just the def site scope though. I think we can mention that macro_rules are scoped differently

[Manishearth]

Added a note. We'll be deprecating macro_rules anyway so I don't think we should do much more here

[GuillaumeGomez]

Please remove this empty line.

[GuillaumeGomez]

Overall, it is nice. Could you also add the fact that using crate:: or paths more generally work just like imports too please?