Add documtation for the rustfmt::skip attribute

[Documentation](https://rust-lang.github.io/rustfmt/?version=v1.8.0) for the `#[rustfmt::skip]` attribute does not exist. The `#[rustfmt::skip::macros(target_macro_name)]` and `#[rustfmt::skip::attributes(target_attribute_name)]` variants are only mentioned in the README and it is unclear if there are more.