Greatly improve E0322 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0322.md

@@ -1,3 +1,13 @@
+The `Sized` trait was implemented explicitly.
+
+Erroneous code example:
+
+```compile_fail,E0322
+struct Foo;
+
+impl Sized for Foo {} // error!
+```
+
 The `Sized` trait is a special trait built-in to the compiler for types with a
 constant size known at compile-time. This trait is automatically implemented
 for types as needed by the compiler, and it is currently disallowed to