doc: child_process corrections and cleanups

- Make explicit that .disconnected is set before the disconnect event,
  and it is not allowed to send messages after calling .disconnect(),
  even while waiting for a delayed disconect event.
- Remove obsolete claim that explicit exit is required
- Describe silent: in the options for fork()
- Describe .connected as the property it is, not just as an aside in
  the disconnect() method