feat(prefer-expect-assertions): adding documetation examples

Author: Mario Campa

docs/rules/prefer-expect-assertions.md

@@ -69,3 +69,28 @@ When `true`, this rule will only warn for tests that use the `async` keyword.
   }
 }
 ```
+
+When `asyncOnly` option is set to `true`, the following pattern would be a
+warning:
+
+```js
+test('my test', async () => {
+  const result = await someAsyncFunc();
+  expect(result).toBe('foo');
+});
+```
+
+While the following patterns would not be considered warnings:
+
+```js
+test('my test', () => {
+  const result = someFunction();
+  expect(result).toBe('foo');
+});
+
+test('my test', async () => {
+  expect.assertions(1);
+  const result = await someAsyncFunc();
+  expect(result).toBe('foo');
+});
+```