docs-bug(CdkOverlay): Overlay scroll strategies do not work unless main body is the scroll

### Documentation Feedback

The documentation for CdkOverlay doesn't mention the ScrollStrategies only work if the `window` object is the one scrolling. As explained in this bug report: #6157 in order to make it work for other scrollable elements, one needs to use the `cdk-scrollable` attribute, but this is not mentioned in the documentation.

Could you add an explanation of it there?

Also, I notice even setting `cdk-scrollable` doesn't make the `block` strategy work with non-window scrolls. So I think even more documentation is missing about how to make all this work.

### Affected documentation page

https://material.angular.io/cdk/overlay/api

https://material.angular.io/cdk/overlay/api の影響を受ける CDK Overlay API ドキュメントページから始め、issue #6157 に記載された動作を確認してください。ユーザーが各スクロール戦略の動作を理解できるように、cdk-scrollable の要件と、ウィンドウ以外のスクロールにおける block strategy の制限事項を文書化してください。

索引モデルが issue の本文から書いたものです。