add doc strings to commands missing them

Change-Id: Ib0320cc4653ff695a7ee2bad3486eeec5309d3b1

secop_psi/trinamic.py

@@ -366,6 +366,7 @@ class Motor(PersistentMixin, HasIodev, Drivable):
 
     @Command(FloatRange())
     def set_zero(self, value):
+        """adapt zero to make current position equal to given value"""
         raw = self.read_value() - self.zero
         self.write_zero(value - raw)