Clarify repr(rust) list is about structs only

Author: Darksonn

src/type-layout.md

@@ -164,7 +164,7 @@ r[layout.repr.rust.intro]
 The `Rust` representation is the default representation for nominal types without a `repr` attribute. Using this representation explicitly through a `repr` attribute is guaranteed to be the same as omitting the attribute entirely.
 
 r[layout.repr.rust.layout]
-The only data layout guarantees made by this representation are those required for soundness. They are:
+The only data layout guarantees made by this representation are those required for soundness. For structs these are:
 
  1. The fields are properly aligned.
  2. The fields do not overlap.