enum and union are unclear

Author: RalfJung

Diff for: reference/src/glossary.md

@@ -192,6 +192,9 @@ for all values `v` and lists of bytes `b` such that `v` and `b` are related at `
 changing `b` at index `i` to any other byte yields a `b'` such `v` and `b'` are related (`Vrel_T(v, b')`).
 In other words, the byte at index `i` is entirely ignored by `Vrel_T` (the value relation for `T`), and two lists of bytes that only differ in padding bytes relate to the same value(s), if any.
 
+This definition works fine for structs.
+The desired notion of "padding byte" for enums and unions is still unclear.
+
 #### Place
 
 A *place* (called "lvalue" in C and "glvalue" in C++) is the result of computing a [*place expression*][place-value-expr].