PeakPickerHiRes docu updated

The documentation for PeakPickerHiRes paramters is updated. As @hendrikweisser pointed out, the descriptions of missing, spacing_difference and spacing_difference_gap were not clear.

min_distance is defined as the smaller of the two spacing to the left and right from the seed. This distance serves as scale. Small gaps (> spacing_difference * min_distance) define a missing data point and are allowed (if missing > 0). Large gaps (> spacing_difference_gap * min_distance) immediately lead to an end of the peak extension.

IMO, all three parameters require the data to be roughly equidistant. Otherwise the concept of 'missing data point' or 'large gap' is ill defined.