fix command doc string handling and change default stop doc string

- fix inheritance of command description
- when no stop method is given, then the description should indicate
  that stop is a no-op -> add missing doc strings to stop methods
- add test to make sure stop command doc strings are given
  when implemented

Change-Id: If891359350e8dcdec39a706841d61d4f8ec8926f
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/33266
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy/lib/sequence.py

@@ -141,6 +141,7 @@ class SequencerMixin:
         return self.Status.IDLE, ''
 
     def stop(self):
+        """stop sequence"""
         if self.seq_is_alive():
             self._seq_stopflag = True