Rollup merge of rust-lang#69311 - GuillaumeGomez:clean-up-e0321-e0322, r=Dylan-DPC

Clean up E0321 and E0322

r? @Dylan-DPC

Author: Centril

src/librustc_error_codes/error_codes/E0321.md

@@ -1,5 +1,7 @@
 A cross-crate opt-out trait was implemented on something which wasn't a struct
-or enum type. Erroneous code example:
+or enum type.
+
+Erroneous code example:
 
 ```compile_fail,E0321
 #![feature(optin_builtin_traits)]

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