Updated documentation to reflect the actual behavoir of the code.

islandora.api.php

@@ -638,7 +638,8 @@ function hook_CMODEL_PID_islandora_overview_object_alter(AbstractObject &$object
  *     module_name_derivative_creation_function($object, $force = FALSE)
  *     These functions must return an array in the structure of:
  *     - success: Bool denoting whether the operation was successful.
- *     - messages: An array structure containing:
+ *     - messages: An array structure containing zero or more array's with the
+ *       following fields:
  *       - message: A string passed through t() describing the
  *         outcome of the operation.
  *       - message_sub: (Optional) Substitutions to be passed along to t() or