rustdoc: warn for `notrust` tags

Rustdoc should warn when `notrust` is used. The tag can be replaced by an actual code marker like `c`, `plain`, `console` etc.

One outstanding issue is that `rustdoc` has no default log level thus does not print the warning in normal use. Is there a good way to raise that?

Following the discussion here:

https://github.com/rust-lang/rfcs/issues/500