rustdoc: Document exported macros

[sfackler]

Now that macros can be exported across crates, it'd be nice if Rustdoc would pick up macro_rules-style macros that are exported. E.g.

/// This is a macro that foos
#[macro_export]
macro_rules! foo(
    (slow $i:ident) => (...);
    (fast $i:ident) => (...);
)

Bonus points for being able to document each pattern.

[huonw]

Closing as a dupe of #9954.