Intra-doc links don't accept all macro delimiters

Consider this code:

```rust
// This can be invoked with `foo!()`, `foo[]` or `foo{}`
#[macro_export] macro_rules! foo {
    () => {}
}

/// This is parsed as an intra-doc link: [foo!()]...
/// ...but those aren't: [foo![]] [foo!{}]
pub struct Foo;
```

I'm not sure if this is a deliberate decision or not. If it is, what's the rationale for not accepting `[]` and `{}`? cc @jyn514