Document that match guards are "distributed" across mutliple patterns

[HMPerson1]

Example function (playground):

fn or(b1: bool, b2: bool) -> bool {
    match (b1, b2) {
        (b, _) | (_, b) if b => true,
        _ => false,
    }
}

When we call or(false, true), it matches the (b, _) pattern, then runs the guard which fails, then matches the (_, b) pattern in the same arm and runs the guard again which succeeds, and then finally returns true.

With the current documentation (book, refererence), it's not clear that after matching (b, _) and failing the guard, it will try to match the (_, b) pattern in the same arm instead of failing the whole arm proceeding to the next arm (_ => false).

See also rust-lang/rust#26998, rust-lang/rust#26012.

(moved from rust-lang/rust#51745)