add more details to getGridSize() docstring

src/omero/gateway/__init__.py

@@ -6645,6 +6645,7 @@ def getGridSize(self):
         """
         Iterates all wells on plate to retrieve grid size as {'rows': rSize,
         'columns':cSize} dict.
+        If the Plate has no Wells, it will return {'rows': 0, 'columns': 0}
 
         :rtype:     dict of {'rows': rSize, 'columns':cSize}
         """