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

Comience con la página de documentación de la API de CDK Overlay afectada en https://material.angular.io/cdk/overlay/api y revise el comportamiento descrito en el issue #6157. Documente los requisitos de cdk-scrollable y las limitaciones de la block strategy con desplazamiento que no sea de ventana, para que los usuarios puedan entender cómo funciona cada estrategia de desplazamiento.

Escrito por el modelo de indexación a partir del texto del issue.