as_ref suggestions should warn about UB concerns

[RalfJung]

With #61444 we are now suggesting people to use as_ref when they have a raw pointer and need a reference. With #61785 as_ref gets some expanded documentation on UB avoidance. It might be a good idea to mention that also in the actual suggestion, so people don't have to open the docs to be aware of this.

Cc @estebank

[imbrem]

Can I take this on, since no one seems to have done it yet?

[RalfJung]

Sure, it's yours!