Document RFC 2539 (`#[cfg_attr(p, attr1, attr2)]`)

- Version: 1.33
- PR: https://github.com/rust-lang/rust/pull/57332
- Tracking issue: https://github.com/rust-lang/rust/issues/54881
- RFC: https://github.com/rust-lang/rfcs/pull/2539