Update comment on pointer-to-usize transmute

Co-authored-by: Ralf Jung <post@ralfj.de>

library/core/src/ptr/mod.rs

@@ -1652,9 +1652,10 @@ pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
 
     let stride = mem::size_of::<T>();
 
-    // SAFETY: At runtime, transmuting a pointer to `usize` is always safe, because they have the
-    // same layout. During const eval, we hook this function to ensure that the pointer always has
-    // an address (only the standard library can do this).
+    // SAFETY: This is just an inlined `p.addr()` (which is not
+    // a `const fn` so we cannot call it).
+    // During const eval, we hook this function to ensure that the pointer never
+    // has provenance, making this sound.
     let addr: usize = unsafe { mem::transmute(p) };
 
     // SAFETY: `a` is a power-of-two, therefore non-zero.