Zero Sized Types: stop saying that "dereferencing" a non-ZST is UB

From discussion at
https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/What.20operations.20are.20allowed.20on.20ZST.20null.20pointers.3F

 - applying the dereference operator to a null pointer stopped being UB
   a few months ago

 - it's better to be explicit about whether we're talking about loads
   and stores or about applying the dereference operator

I think we don't need to mention field projections here.

Author: mattheww

src/exotic-sizes.md

@@ -106,9 +106,9 @@ consequence of types with no size. In particular, pointer offsets are no-ops,
 and allocators typically [require a non-zero size][alloc].
 
 Note that references to ZSTs (including empty slices), just like all other
-references, must be non-null and suitably aligned. However, dereferencing a
-null pointer to a ZST is not [undefined behavior][ub], unlike pointers to
-other types.
+references, must be non-null and suitably aligned. However, loading or storing
+through a null pointer to a ZST is not [undefined behavior][ub], unlike
+pointers to other types.
 
 [alloc]: ../std/alloc/trait.GlobalAlloc.html#tymethod.alloc
 [ub]: what-unsafe-does.html