[docs] More info on terminating pipelines gracefully

[kidq330]

I've had trouble searching through the docs for info on how to gracefully terminate a pipeline after all its elements received an EOS. Example:

Mix.install([
  {:membrane_generator_plugin, ">= 0.0.0"},
  {:membrane_core, "~> 1.0"}
])

defmodule MyPipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_ctx, _opts) do
    spec = [
      child(
        :generator,
        %Membrane.SilenceGenerator{
          stream_format: %Membrane.RawAudio{
            channels: 1,
            sample_rate: 16_000,
            sample_format: :s16le
          },
          duration: Membrane.Time.milliseconds(100)
        }
      )
      |> child(:sink, Membrane.Fake.Sink)
    ]

    {[spec: spec], nil}
  end
end

{:ok, supervisor, pipeline} = Membrane.Pipeline.start_link(MyPipeline, [])
Process.monitor(supervisor)

defmodule Loop do
  def loop() do
    receive do
      {:DOWN, _ref, :process, _pid, _reason} ->
        IO.puts("Pipeline down")
      other ->
        IO.inspect(other, label: "received unknown message")
    end

    loop()
  end
end

Loop.loop()

The solution was to include def handle_element_end_of_stream(:sink, _ctx, state), do: {[terminate: :normal], state} - this would be triggered when the sink receives an EOS, which for this simple case would mean :end_of_stream messages covered the entire pipeline. This might not be the case for more complex pipeline topologies. I was under the impression the framework would automatically detect and terminate when all elements receive EOS, so a section in the guides going through the process of adding termination mechanisms would provide DX value.