Correct `lightning-net-tokio` documentation, remove stale example

Correct `lightning-net-tokio` documentation, remove stale example

The `lightning-net-tokio` crate-level example contained a carryover
from when it was the primary notifier of the background processor
and now just shows an "example" of creating a method to call
another method with the same parameters and then do event
processing (which doesn't make sense, the BP should do that).

Instead, the examples are simply removed and the documentation is
tweaked to include recent changes.