Clean up E0374 explanation

src/librustc_error_codes/error_codes/E0374.md

@@ -1,9 +1,5 @@
-A struct without a field containing an unsized type cannot implement
-`CoerceUnsized`. An [unsized type][1] is any type that the compiler
-doesn't know the length or alignment of at compile time. Any struct
-containing an unsized type is also unsized.
-
-[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+`CoerceUnsized` was implemented on a struct which does not contain a field with
+an unsized type.
 
 Example of erroneous code:
 
@@ -20,6 +16,12 @@ impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
     where T: CoerceUnsized<U> {}
 ```
 
+An [unsized type][1] is any type where the compiler does not know the length or
+alignment of at compile time. Any struct containing an unsized type is also
+unsized.
+
+[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+
 `CoerceUnsized` is used to coerce one struct containing an unsized type
 into another struct containing a different unsized type. If the struct
 doesn't have any fields of unsized types then you don't need explicit