Document lint control scoping.

[ehuss]

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.
 }
}

[ehuss]

rust-lang/rust#77534 changed the behavior of overriding the same lint in the same scope. The rules for that should also be included.