Clipboard code example in Material 9.0.0 docs does not work on Firefox

#### Documentation Feedback

The following code example from the Clipboard documentation for Angular Material 9.0.0 does not work on Firefox:

https://github.com/angular/components/blame/a67cef6cf048166ebd1934637dbe0136c2dd345c/src/cdk/clipboard/clipboard.md#L43

The problem seems to be that Firefox does not allow Javascript applications to write to the clipboard except synchronously inside a click handler (unless special permissions have been granted):

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Interact_with_the_clipboard#Writing_to_the_clipboard

Therefore, the fact that the `attempt` function is called asynchronously using `setTimeout` causes the call to fail, with a console warning message saying the copy was "denied because it was not called from inside a short running user-generated event handler".

It's unclear to me why the example code is written with the copy attempts done asynchronously. Running the attempts inline seems to work fine, even for a fairly substantial amount of data:

```typescript
let clipboardData = 'something to copy';
const pending = this.clipboard.beginCopy(clipboardData);
for (let attempt = 0; attempt < 3; attempt++) {
const result = pending.copy();
if (result) {
console.log('Copied on attempt ' + (attempt+1));
break;
}
}
pending.destroy();
```

**Affected documentation page:**
https://material.angular.io/cdk/clipboard/overview

Bắt đầu với src/cdk/clipboard/clipboard.md quanh dòng 43 được liên kết và so sánh các lần thử không đồng bộ với setTimeout với ví dụ inline trong báo cáo. Cập nhật ví dụ trong tài liệu Clipboard để nó hoạt động với Firefox, sau đó xác minh ví dụ trên trang tổng quan Clipboard bị ảnh hưởng và trong Firefox.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.