Inline HTTP crate docs into core lib

[SergioBenitez]

Blocked by rust-lang/rust#51252.

[QuietMisdreavus]

A PR that fixed that issue was just merged, so the next nightly that comes out should have it.

[SergioBenitez]

Excellent! This has indeed been fixed. Thanks, @QuietMisdreavus!

[SergioBenitez]

Note that hidden items are currently being reexported in the http docs due to rust-lang/rust#53417.