Add explanation message for E0203

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diff --git a/src/librustc_error_codes/error_codes/E0203.md b/src/librustc_error_codes/error_codes/E0203.md
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+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`