Improve E0184 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0184.md

@@ -1,6 +1,20 @@
-Explicitly implementing both Drop and Copy for a type is currently disallowed.
-This feature can make some sense in theory, but the current implementation is
-incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
-it has been disabled for now.
+The `Copy` trait was implemented on a type with a `Drop` implementation.
+
+Erroneous code example:
+
+```compile_fail,E0184
+#[derive(Copy)]
+struct Foo; // error!
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+    }
+}
+```
+
+Explicitly implementing both `Drop` and `Copy` trait on a type is currently
+disallowed. This feature can make some sense in theory, but the current
+implementation is incorrect and can lead to memory unsafety (see
+[issue #20126][iss20126]), so it has been disabled for now.
 
 [iss20126]: https://github.com/rust-lang/rust/issues/20126