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

从受影响的 CDK Overlay API 文档页面 https://material.angular.io/cdk/overlay/api 开始，并检查 issue #6157 中描述的行为。记录 cdk-scrollable 的要求，以及 block strategy 在非窗口滚动场景下的限制，以便用户了解每种滚动策略的工作方式。

由索引模型根据 Issue 内容生成。