Clarify that any alignment is valid for ZSTs

src/repr-rust.md

@@ -11,10 +11,10 @@ Primitives are usually aligned to their size, although this is
 platform-specific behavior. For example, on x86 `u64` and `f64` are often
 aligned to 4 bytes (32 bits).
 
-A type's size must always be a multiple of its alignment. This ensures that an
-array of that type may always be indexed by offsetting by a multiple of its
-size. Note that the size and alignment of a type may not be known
-statically in the case of [dynamically sized types][dst].
+A type's size must always be a multiple of its alignment (Zero being a valid size
+for any alignment). This ensures that an array of that type may always be indexed
+by offsetting by a multiple of its size. Note that the size and alignment of a
+type may not be known statically in the case of [dynamically sized types][dst].
 
 Rust gives you the following ways to lay out composite data: