message: Add note about side effects of drop
diff: @@ -137,6 +137,10 @@ return an input. The next two functions drop unexpected messages with or
  without printing a warning beforehand. Finally, ``skip`` leaves the
  input message in the mailbox. The default is ``print_and_drop``.
  +*Note:* ``print_and_drop`` and ``drop`` return an error message that is
  +delivered to the sender of the unexpected message. If that actor does not have
  +an explicit handler for error messages it will terminate.
  +
  .. _request:
  Requests
  