Clarify when `TryFromBytes` and `IntoBytes` aren't symmetrical

[mkeeter]

The docs for TryFromBytes say

Code should not generally assume that values produced by IntoBytes will necessarily be accepted as valid by TryFromBytes.

It would be helpful to explain cases where these traits are asymmetrical, so that people could use TryFromBytes with confidence. As written, it seems like even trivial code is of unknown correctness, e.g.

use zerocopy::{IntoBytes, TryFromBytes, Immutable, KnownLayout};

#[derive(Debug, PartialEq, TryFromBytes, IntoBytes, Immutable, KnownLayout)]
struct Foo {
    hello: u32,
    world: u32,
}

fn main() {
    let f = Foo {
        hello: 0x1234,
        world: 0x5678,
    };
    let bytes = f.as_bytes();
    let f_ = Foo::try_ref_from_bytes(bytes).expect("is this okay to do?");
    assert_eq!(&f, f_);
}