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

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.

```rust
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_);
}
```