demo: add docstring to stop method of sampletemp

Change-Id: Ic575189eaf828d082a7ac382ad38141458c31522
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/37183
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>

frappy_demo/modules.py

@@ -249,6 +249,7 @@ class SampleTemp(Drivable):
         # note: we may also return the read-back value from the hw here
 
     def stop(self):
+        """Stop and keep temperature at current position."""
         self.target = self.value
 
     def _thread(self):