rustdoc now supports compiling things with `--test` so the examples in
this guide can be compiled & tested properly (revealing a few issues &
out-dated behaviours).
Also, reword an example to be clearer, cc #12242.
Now that rustdoc understands proper language tags
as the code not being Rust, we can tag everything
properly.
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.
The changes are basically just because rustdoc runs tests/rendering on
more snippets by default (i.e. everything without a `notrust` tag), and
not anything significant.
