docs: add sync-only info to expect.toThrowError tip (#7381)

Author: flx-sta

docs/api/expect.md

@@ -701,6 +701,14 @@ You can provide an optional argument to test that a specific error is thrown:
 
 :::tip
 You must wrap the code in a function, otherwise the error will not be caught, and test will fail.
+
+This does not apply for async calls as [rejects](#rejects) correctly unwraps the promise:
+```ts
+test('expect rejects toThrow', async ({ expect }) => {
+  const promise = Promise.reject(new Error('Test'))
+  await expect(promise).rejects.toThrowError()
+})
+```
 :::
 
 For example, if we want to test that `getFruitStock('pineapples')` throws, we could write: