docs: update proposal to reflect polymorphic expectFailure API

Author: Han5991

proposals/expect-failure-enhancements.md

@@ -21,19 +21,28 @@ test('fails with a specific reason', {
 - **Validation**: None. It accepts *any* error.
 - **Output**: The reporter displays the string (e.g., `# EXPECTED FAILURE Bug #123...`).
 
-### 2. RegExp: Error Matcher (via Object)
-Use the object form with the `with` property.
+### 2. Validation (RegExp, Class, Function, Object)
+If the value is a **RegExp**, **Class**, **Function**, or an **Object** (without a `with` property), it is treated as an error matcher.
 
 ```js
-test('fails with matching error', {
-  expectFailure: { with: /expected error message/ }
+test('fails with matching regex', {
+  expectFailure: /expected error message/
 }, () => {
-  throw new Error('this is the expected error message');
+  throw new Error('expected error message');
+});
+
+test('fails with matching class', {
+  expectFailure: TypeError
+}, () => {
+  throw new TypeError('wrong type');
 });
 ```
+- **Behavior**: The test passes **only if** the thrown error matches the criteria.
+- **Validation**: Uses `assert.throws` logic internally.
+- **Output**: Reporter shows generated message (e.g. `# EXPECTED FAILURE matching /regex/`).
 
-### 3. Object: Reason & Validation
-When an **Object** is provided, it allows specifying both a failure reason and validation logic simultaneously.
+### 3. Explicit Configuration Object
+To provide **both** a reason and validation, use an object with `message` and `with` properties.
 
 ```js
 test('fails with reason and specific error', {
@@ -46,15 +55,14 @@ test('fails with reason and specific error', {
 });
 ```
 - **Properties**:
-    - `message` (String): The failure reason/label (displayed in reporter).
-    - `with` (RegExp | Object | Function | Class): Validation logic. This is passed directly to `assert.throws` validation argument, supporting all its capabilities.
-- **Behavior**: The test passes **only if** the error matches the `with` criteria.
-- **Output**: The reporter displays the `message`.
+    - `message` (String): The failure reason/label.
+    - `with` (Any): Validation logic passed to `assert.throws`.
 
 ## Ambiguity Resolution
-Potential ambiguity is resolved by strict type separation:
+Potential ambiguity is resolved by checking the type and structure:
 *   `typeof value === 'string'` → **Reason**
-*   `typeof value === 'object'` → **Configuration Object** (`message` and/or `with`)
+*   `typeof value === 'object' && value.with !== undefined` → **Explicit Config**
+*   Otherwise (`RegExp`, `Function`, `Object`) → **Validation Matcher**
 
 ## Alternatives Considered