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

リンク先の43行目付近にある src/cdk/clipboard/clipboard.md から始め、非同期の setTimeout の試行とレポート内のインライン例を比較します。Clipboard のドキュメント例を更新して Firefox で動作するようにし、その後、影響を受ける Clipboard 概要ページと Firefox で例を検証します。

索引モデルが issue の本文から書いたものです。