Extend the guarantee to all struct types

Author: gnzlbg

reference/src/layout/structs-and-tuples.md

@@ -124,16 +124,17 @@ x: u16, y: T }` where `T = u32` are not guaranteed to be identical.
 
 #### Zero-sized structs
 
-Structs with default layout or default layout with increased alignment are
-zero-sized, if they contain no fields of non-zero size. That is, either the type
-has no fields, or all of its fields have zero size.
+Structs with default layout (`repr(Rust)`), layout with increased alignment
+(`repr(align(N))`), packed layout (`repr(packed(N))`), or C-compatible layout
+(`repr(C)`) are zero-sized, if they contain no fields of non-zero size. That is,
+either the type has no fields, or all of its fields have zero size.
 
 For example, all these types are zero-sized:
 
 ```rust
 # use std::mem::size_of;
 #[repr(align(32))] struct Zst0;
-struct Zst1(Zst0);
+#[repr(C)] struct Zst1(Zst0);
 struct Zst2(Zst1, Zst0);
 # fn main() {
 # assert_eq!(size_of::<Zst0>(), 0);