Rollup merge of rust-lang#125021 - joshlf:patch-11, r=RalfJung

Update reference safety requirements

Per rust-lang#116677 (comment), the language as written promises too much. This PR relaxes the language to be consistent with current semantics. If and when rust-lang#117945 is implemented, we can revert to the old language.

While we're here, we also require that references be non-null.

cc ``@RalfJung``

library/core/src/primitive_docs.rs

@@ -1467,8 +1467,9 @@ mod prim_usize {}
 /// For all types, `T: ?Sized`, and for all `t: &T` or `t: &mut T`, when such values cross an API
 /// boundary, the following invariants must generally be upheld:
 ///
+/// * `t` is non-null
 /// * `t` is aligned to `align_of_val(t)`
-/// * `t` is dereferenceable for `size_of_val(t)` many bytes
+/// * if `size_of_val(t) > 0`, then `t` is dereferenceable for `size_of_val(t)` many bytes
 ///
 /// If `t` points at address `a`, being "dereferenceable" for N bytes means that the memory range
 /// `[a, a + N)` is all contained within a single [allocated object].