Document use _ as _ syntax

[sinistersnare]

/u/DroidLogician on reddit mentioned that the use _ as _ syntax is undocumeted, at least in the crates and module chapter of TRPL.

@steveklabnik, this seems like your territory :)

[abonander]

Also the extern "crate" as my_crate; syntax.

[daboross]

As mentioned later in that reddit thread, extern crate "crate" as my_crate; is now extern crate cratename as my_crate; without quotes. "crate" hasn't worked since the addition of turning - into _ in crate names.

[durka]

I think this can be closed.