Document `#[repr(packed(N))]`

[Centril]

• Version: 1.33
• PR: Stabilize #[repr(packed(N))] rust#57049
• Tracking issue: #[repr(packed(N))] (tracking issue for RFC 1399) rust#33158
• RFC: RFC: Add #[repr(pack = "N")] rfcs#1399

The implementation diverged from the RFC a bit...

cc @cramertj