Added missing code changes on GenStage documentation #210 (#211)

Author: alvaromlg

lib/gen_stage.ex

@@ -241,7 +241,21 @@ defmodule GenStage do
       GenStage.start_link(C, :ok)
       GenStage.start_link(C, :ok)
 
-  In a supervision tree, this is often done by starting multiple workers:
+  In a supervision tree, this is often done by starting multiple workers. Typically
+  we update each `c:start_link/1` call to start a named process:
+
+      def start_link(number) do
+        GenStage.start_link(A, number, name: A)
+      end
+
+  And the same for module `B`:
+
+      def start_link(number) do
+        GenStage.start_link(B, number, name: B)
+      end
+
+  Module `C` does not need to be updated because it won't be subscribed to.
+  Then we can define our supervision tree like this:
 
       children = [
         worker(A, [0]),
@@ -254,9 +268,9 @@ defmodule GenStage do
 
       Supervisor.start_link(children, strategy: :rest_for_one)
 
-  Having multiple consumers is often the easiest and simplest way to
-  leverage concurrency in a GenStage pipeline, especially if events can
-  be processed out of order.
+  Having multiple consumers is often the easiest and simplest way to leverage
+  concurrency in a GenStage pipeline, especially if events can be processed out
+  of order.
 
   Also note that we set the supervision strategy to `:rest_for_one`. This
   is important because if the producer A terminates, all of the other