document more about bidirectional configurations

[vic]

In den it is possible for a host aspect to contribute configuration to each user homeManager, also user aspects can contribute nixos options to their host. This is mentioned already in the docs (as bidirectional dependencies) but it would be nice to have examples about how this is useful :

• a host aspect can detect if two users are present, if they are, provide config to each of their homeManager.

• a user aspect detects features of its host and configures the host (somewhat already exemplified with user being configured as admin)

any other interesting cases?