Be explicit with rustdoc.

I missed some annotations, and some were in a different style.
librustc: Obsolete the old external crate renaming syntax.

Instead of `extern crate foo = bar`, write `extern crate bar as foo`.
Instead of `extern crate baz = "quux"`, write `extern crate "quux" as
baz`.

Closes #16461.

[breaking-change]
auto merge of #17048 : pcwalton/rust/remove-old-import-renaming-syntax, r=brson

Instead of `extern crate foo = bar`, write `extern crate bar as foo`.
Instead of `extern crate baz = "quux"`, write `extern crate "quux" as
baz`.

Closes #16461.

[breaking-change]

r? @brson
auto merge of #17108 : steveklabnik/rust/explicitness, r=alexcrichton

I missed some annotations, and some were in a different style.