Clean up E0164 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0164.md

@@ -1,24 +1,44 @@
-This error means that an attempt was made to match a struct type enum
-variant as a non-struct type:
+Something which is neither a tuple struct nor a tuple variant was used as a
+pattern.
+
+Erroneous code example:
 
 ```compile_fail,E0164
-enum Foo { B { i: u32 } }
+enum A {
+    B,
+    C,
+}
+
+impl A {
+    fn new() {}
+}
 
-fn bar(foo: Foo) -> u32 {
+fn bar(foo: A) {
     match foo {
-        Foo::B(i) => i, // error E0164
+        A::new() => (), // error!
+        _ => {}
     }
 }
 ```
 
-Try using `{}` instead:
+This error means that an attempt was made to match something which is neither a
+tuple struct nor a tuple variant. Only these two elements are allowed as
+pattern:
 
 ```
-enum Foo { B { i: u32 } }
+enum A {
+    B,
+    C,
+}
+
+impl A {
+    fn new() {}
+}
 
-fn bar(foo: Foo) -> u32 {
+fn bar(foo: A) {
     match foo {
-        Foo::B{i} => i,
+        A::B => (), // ok!
+        _ => {}
     }
 }
 ```