Document single consumer's demand never exceeded (#217)

Adapted from https://elixir-lang.org/blog/2016/07/14/announcing-genstage/

Author: davidsulc

lib/gen_stage/dispatchers/broadcast_dispatcher.ex

@@ -2,6 +2,9 @@ defmodule GenStage.BroadcastDispatcher do
   @moduledoc """
   A dispatcher that accumulates demand from all consumers
   before broadcasting events to all of them.
+  
+  This dispatcher guarantees that events are dispatched to all
+  consumers without exceeding the demand of any given consumer.
 
   If a producer uses `GenStage.BroadcastDispatcher`, its subscribers
   can specify an optional `:selector` function that receives the event