auto merge of #14601 : skade/rust/remove-notrust-tags, r=alexcrichton

Now that rustdoc understands proper language tags
as the code not being Rust, we can tag everything
properly. `norust` as a negative statement is a bad
tag.

This change tags examples in other languages by
their language. Plain notations are marked as `text`.
Console examples are marked as `console`.

Also fix markdown.rs to not highlight non-rust code.

Amends the documentation to reflect the new
behaviour.

diff --cc src/doc/rust.md
Simple merge

diff --cc src/libsyntax/ext/deriving/generic/mod.rs
Simple merge