Document `spin_app::App.id` gurantees

[rylev]

spin_app::App.id is used in the Spin codebase almost exclusively for logging. The only place where it somewhat matters what the id is is in the Redis trigger where it used for telemetry. I feel somewhat uncomfortable that the id is somewhat assumed to be unique to the application, but it's not documented anywhere. Perhaps we want to be clearer about what gurantees users of the id can be sure of, and what callers of spin_app::App::new should try to maintain.