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
doc: cluster documentation cleanup and corrections

- fixed some incomprehensible wording ("event assigned to..."?)
- removed undocumented and unnecessary process properties from example
- corrected the docs on the default for the exec setting
- described when workers are removed from cluster.workers
- described addressType, which was documented as existing, but not what
  values it might have
- spell out more clearly the limitations of setupMaster
- describe disconnect in sufficient detail that why a child does or does
  not exit can be understood
- clarify which cluster functions and events are available on process or
  just on the worker, as well as which are not available in children,
- don't describe events as the same, when they have receive different
  arguments
- fix misleading disconnect example: since disconnect already calls
  close on all servers, doing it again in the example is a no-op, not
  the "force close" it was claimed to be
- document the error event, not catching it will kill your node
- describe suicide better, it is important, and a bit unintuitive
  (process.exit() is not suicide?)
- use worker consistently throughout, instead of child.