Documentation for `calculate_percentile_cut` unclear for case of underflows

[Tobychev]

The API for calculate_percentile_cut contains both a min_value argument and a fill_value argument, both of which claim to be the value which will be assigned to events with values smaller than min_value.

Looking at the code it seems it is only the the value of fill_value that is used, but I'm honestly not sure of how this part is supposed to work.

[maxnoe]

Hi,

I think the documentation is correct and in line with the behavior, but it probably could be more clear.

The doc strings state:

    fill_value: float or quantity
        Value for bins with less than ``min_events``,
        must have same unit as values
    min_value: float or quantity or None
        If given, cuts smaller than this value are replaced with ``min_value``

So both specify a value to replace the computed value, but under different conditions:

fill_value is used when the number of events in the bin is less than the required min_events and min_value is a simple clipping value for the computed value itself.

[maxnoe]

The code is here:

https://github.com/cta-observatory/pyirf/blob/4b617cc678ee83dc283c6d7e5f87e8a2da566839/pyirf/cuts.py#L77-L84

It matches the documentation (fill_value used if n_events < min_events, and min/max value uses as arguments to np.clip)