add document
	modified:   lib/system_elixir/event.ex
	modified:   lib/system_elixir/log.ex
	modified:   lib/system_elixir/process.ex
	modified:   lib/system_elixir/signal.ex
	modified:   lib/system_elixir/simtime.ex
	modified:   lib/system_elixir/simulate.ex