expand description of the link attribute

Fixes #18288 

I'm not sure if the entire information from the FFI guide should go here or not. http://doc.rust-lang.org/nightly/guide-ffi.html#linking

I'm thinking maybe that should all go in the reference. This is just the smallest change. What do you think, @huonw ?
Yeah, I guess I just assumed that since this entire section was about extern blocks, it seemed gratuituos