Document lint control scoping. #574

The diagnostics chapter should explain how allow/deny/etc works when the same rule is applied at different levels. For example:

#[allow(unused)]
{
    #[deny(unused)]
    {
        // Everything here is deny.
    }
}

This is pretty self-evident, but the docs should be explicit how this scoping works (it's fairly unusual attribute behavior).

Also, it should probably mention lint groups. For example:

#[allow(unused)]
{
    #[deny(unused_variables)]
    {
        // all unused lints are allow, except unused_variables.
    }
}