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

Start with the affected CDK Overlay API documentation page at https://material.angular.io/cdk/overlay/api and review the behavior described in issue #6157. Document the requirements for cdk-scrollable and the limitations of the block strategy with non-window scrolling, so users can understand how each scroll strategy works.

Written by the indexing model from the issue text.