update compute_fill_first_cost_fn docstring
update compute_fill_first_cost_fn docstring

The default value for weight of the compute_fill_first_cost_fn is
actually -1.0, which means it does exactly the opposite of what
the function docstring says it does. Update the docstring to
warn people of this fact.

Issue reported via bug #1023827 which was closed as won't fix
but the docs should be clearer. Updated based on feedback.

Change-Id: I56acde254c932a8e66f941aaeb7304665500610f

n0ano/gantt,n0ano/gantt