Add to_pyat docstring to Synchrotron class

mbtrack2/tracking/synchrotron.py

@@ -114,6 +114,8 @@ class Synchrotron:
     get_longitudinal_twiss(V)
         Compute the longitudinal Twiss parameters and the synchrotron tune for 
         single or multi-harmonic RF systems.
+    to_pyat(Vrf)
+        Return a pyAT simple_ring element from the Synchrotron element data.
     """
     def __init__(self, h, optics, particle, **kwargs):
         self._h = h