[TC11.50]cm_SelectAll*,cm_ClearAll* : add negative parameters to Help

[dvb]

cm_SelectAll*,cm_ClearAll also work with negative parameters.

Maybe add to Help:

n <= -1: Select every nth line upwards from the current position

[sa16]

n>=1: Select every nth line downwards from the current position (n<=-1: upwards)

Tested OK using TC 11.50rc2 (Dialog box: Choose command).