Clarify that the trait is conservative: it's safe to act on if it's true, but
false doesn't necessarily mean anything. Of course this is the only reasonable
way to use it, but I think it's helpful to be explicit, especially because
currently this trait *always* returns false on some platforms even for a type
like int.

PiperOrigin-RevId: 519272294
Change-Id: Ic63a48dcf18efc0756046b3d6f51d11cbb46a469