Picked a name for the struct/enum/union set.

Author: Havvy

src/type-layout.md

@@ -104,11 +104,11 @@ Closures have no layout guarantees.
 
 ## Representations
 
-All **FIXME** types have a *representation* that specifies what the layout
-is for the type.
+All user-defined composite types (`struct`s, `enum`, and `union`s) have a
+*representation* that specifies what the layout is for the type.
 
-Note: The representation does not depend upon the type's fields or generic
-parameters.
+> Note: The representation does not depend upon the type's fields or generic
+> parameters.
 
 The possible representations for a type are the default representation, `C`, the
 primitive representations, and `packed`. Multiple representations can be applied