Document if_while_or_patterns

RFC: https://github.com/rust-lang/rfcs/pull/2175 + https://github.com/rust-lang/rfcs/pull/2530
Proposal: https://github.com/rust-lang/rust/issues/56212
PR: https://github.com/rust-lang/rust/pull/57532
Tracking: https://github.com/rust-lang/rust/issues/48215
Target = 1.33