Rollup merge of #151892 - chahar-ritik:e0423-enum-doc, r=Kivooeo

Document enum types used as values for E0423

### Problem

The E0423 error explanation did not include an example for enum types being used
as values, which is a common source of confusion for users.

For example, the following code:

```rust
fn main() {
    let x = Option::<i32>;
}
```

compiler/rustc_error_codes/src/error_codes/E0423.md

@@ -44,3 +44,14 @@ 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`
+}
+```