Add explanation message for E0203

src/librustc_error_codes/error_codes/E0203.md

@@ -0,0 +1,12 @@
+Having multiple relaxed default bounds is unsuported.
+
+Erroneous code example:
+
+```compile_fail,E0203
+
+trait Foo {}
+
+struct S5<T>(*const T) where T: ?Foo + ?Sized;
+```
+
+Here the type `T` cannot have a relaxed bound for both `Foo` and `Sized`