Relax slice safety requirements

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

Author: joshlf

library/core/src/primitive_docs.rs

@@ -1468,7 +1468,7 @@ mod prim_usize {}
 /// boundary, the following invariants must generally be upheld:
 ///
 /// * `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].