Document enum types used as values for E0423

compiler/rustc_error_codes/src/error_codes/E0423.md

@@ -44,3 +44,16 @@ fn h1() -> i32 {
     // did you mean `a::I`?
 }
 ```
+
+
+
+### Enum types used as values
+
+Enums are types and cannot be used directly as values.
+
+```compile_fail,E0423
+fn main() {
+    let x = Option::<i32>;
+    //~^ ERROR expected value, found enum `Option`
+}
+```