optional outputs: better document how family triggers work with optional outputs

[oliver-sanders]

Users often struggle when optional outputs and families meet.

We should clarify what familiy-triggers (e.g. :succeed-any) mean and give an example or two.

Here's a valid example of usage:

[scheduling]
    [[graph]]
        R1 = """
            # don't bother running tasks 2&3 unless task 1 succeeded
            task1? => task2? & task3?

            # only run fin if all the tasks succeeded
            FAM:succeed-all? => fin

            # run something_worked if any of the tasks succeeded
            FAM:succeed-any? => something_worked
        """

[runtime]
    [[task1]]
        inherit = FAM
    [[task2]]
        inherit = FAM
    [[task3]]
        inherit = FAM
    [[FAM]]
    [[fin, something_worked]]