No mention of `self` in `extern crate` declarations.

[alercah]

rust-lang/rust#57407 made it possible to refer to self in extern crate _ as _; items to alias the current crate. This is not mentioned on the extern crate page.

[ehuss]

I think this was done in #517. Is there something missing?

[alercah]

Ah, I was looking on the wrong section of the page. Thanks!