Unescaped HTML in unstable reason

[jleedev]

The docs for FnBox (https://doc.rust-lang.org/std/boxed/trait.FnBox.html) contain this line in the rendered output:

will be deprecated if and when Box becomes usable

In the HTML source, this is the unescaped Box<FnOnce> instead of the escaped Box<FnOnce>.

[durka]

rustdoc needs to do the escaping. The reasons are also printed in error messages to the console.