Document macro_rules grammar?

[ehuss]

Should the structure of a macro_rules definition be defined? People writing parsers will likely need to parse it, so it might have some use. However, I can see how it being an "extension" makes it separate.

This could tie in with also defining the exact grammar accepted by each of the fragment kinds.

[eddyb]

We should do it for macro foo {...} definitions, and macro_rules! would differ only slightly.