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.
Maximum allowed difference between points during peak extension, in multiples of the minimal difference between the peak apex and its two neighboring points. If this difference is exceeded a missing point is assumed (see parameter 'missing'). A higher value implies a less stringent peak definition, since individual signals within the peak are allowed to be further apart.