Refine design for VCH deletion API to capture "cascade" information

[zjs]

Currently, deletion is modeled as a simple DELETE request on the VCH resource. The design may need to be adjusted to allow users to clearly indicate whether the operation should "cascade" and include children (e.g., running containers).

[zjs]

Added the proposed design to the overall design document PR (#6124), since that has not been merged yet: cf6578a