message: doc: Update book note to remind myself to document factories
  Need to document async factories since we can't do associated
  functions in async traits.
diff: @@ -54,4 +54,4 @@ _TODO: Talk about how we shuttle futures around inside the library_
  ## Getting Around Associated Functions Like new()
  -_TODO: Talk about non-async constructor traits_
  +_TODO: Talk about non-async constructor traits, factories like DeviceCreatorImpl, etc..._
  