ecore/fd_handler - Fix prepare_callback_set and improve its doc.

ecore/fd_handler - Fix prepare_callback_set and improve its doc.

Now they can be set even if the list is empty (sorry discomfitor,
removing your optimization and making it O(n) again, back from O(0)).

Also notice that due to the already existing check, if a prepare
callback was already set to a fd handler, it can't be changed, so I
added that to the docs.

git-svn-id: http://svn.enlightenment.org/svn/e/trunk/ecore@60765 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33