Add application-level documentation for accessing USB interfaces
It's not clear if this is something that applications will do themselves, or if the phone environment will take care of all USB connections. For example, there is no need to write apps to use peripherals like mice and keyboards, but it might be useful for an app to know when USB is connected if it provides a service that uses USB to communicate.
If flatpak applications will access USB interfaces directly, they will need permission.
More use cases for USB-enabled apps are needed.