Variadic foreign functions are not documented in the reference or TRPL

[frewsxcv]

Implemented in #10064

e.g. https://github.com/rust-lang-nursery/rand/blob/71cba26d4c69eb57b9a10a5917e2fffd49de0b92/src/os.rs#L81

[frewsxcv]

I'm working on this right now.