Update comment for start sequence problem

lib/fluent/root_agent.rb

@@ -136,9 +136,12 @@ def lifecycle(desc: false, kind_callback: nil)
     def start
       lifecycle(desc: true) do |i| # instance
         i.start unless i.started?
-        # Input#start emits lots of evetns with in_tail/`read_from_head true` case and
-        # it causes deadlock for small buffer/queue output. To avoid such problem,
+        # Input#start sometimes emits lots of evetns with in_tail/`read_from_head true` case
+        # and it causes deadlock for small buffer/queue output. To avoid such problem,
         # buffer related output threads should be run before `Input#start`.
+        # This is why after_start should be called immediately after start call.
+        # This depends on `desc: true` because calling plugin order of `desc: true` is
+        # Output, Filter, Label, Output with Router and Input.
         i.after_start unless i.after_started?
       end
     end