rustdoc: Document exported macros

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.

``` rust
/// 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.
