ssconf: update the SetKey docstring

SetKey is used, other than for adding new nodes, in another few cases. Update
the docstring to reflect this, so we don't mislead people reading it.

Reviewed-by: iustinp

lib/ssconf.py

@@ -178,7 +178,8 @@ class SimpleStore:
   def SetKey(self, key, value):
     """Set the value of a key.
 
-    This should be used only when adding a node to a cluster.
+    This should be used only when adding a node to a cluster, or in other
+    infrequent operations such as cluster-rename or master-failover.
 
     """
     file_name = self.KeyToFilename(key)