rustdoc: over-escaped HTML in config tags

[jsha]

https://doc.rust-lang.org/nightly/std/i8/constant.MAX.html shows:

replaced by the <code>MAX</code> associated constant

https://doc.rust-lang.org/std/i8/constant.MAX.html shows:

replaced by the MAX associated constant (with MAX styled by a <code> tag).

/cc @clubby789

[clubby789]

@rustbot claim