Publish docs for `proc_macro` crate

[abonander]

Documentation for the proc_macro crate currently does not appear in nightly builds, at least for Windows. This isn't too big of a deal for now as the API is quite simple, but it would still be nice if this crate appeared in the documentation.

[est31]

Unofficial docs are available at: http://manishearth.github.io/rust-internals-docs/proc_macro/index.html

Still, I wonder how it could be stabilized without making docs available.

[steveklabnik]

Nominating, as this is supposed to be stable in 1.15, and will need a backport.

[steveklabnik]

(Tagging compiler because I imagine that this needs to be un-blacklisted somewhere or something?)

[SimonSapin]

I think it’s a one-liner: #39986