Change description so that the need for values to be positive is better indicated.

cal_tools/cal_tools/agipdlib.py

@@ -81,7 +81,7 @@ class AgipdCorrections:
             writing data out
         :param il_mode: set to true if AGIPD data is interlaced (pre-Nov 2017 data)
         :param karabo_data_mode: set to true to use data iterated with karabo data
-        :param force_hg_if_below: set to a value different to None/0 to force a pixels
+        :param force_hg_if_below: set to a value different to greater 0 to force a pixels
                                   gain to high, if the pixel value is below the given
                                   value after high gain offset subtraction.
         """