rustdoc: html: use raw strings for great justice
rustdoc: add some docs for item types
rustdoc: render derived impls separately
auto merge of #12961 : cmr/rust/rustdoc-impls, r=alexcrichton

Rendered form available at http://docs.octayn.net/doc/

This moves derived impls to the bottom of the list, separate from the rest,
and collapses default methods that aren't overridden into an expandible
accordion.