Document `$crate` in macro matcher

[ehuss]

$crate in a macro matcher is the reserved identifier $crate that doesn't have a way to write it outside of a macro (I think?). That is:

macro_rules! m {
 (x) => {
 m!($crate:tt)
 };
 ($crate:tt) => ()
}

fn main() {
 m!(x); // OK
 m!(y); // ERROR
 m!($crate:tt); // ERROR
 m!(crate:tt); // ERROR
}

[ehuss]

See also #2049 (comment) and #1816 (review). The current docs for the ident matcher is confusing at best in terms of how it accepts $crate.