Explictly state assumptions made by certbot

certbot/interfaces.py

@@ -180,6 +180,9 @@ class IAuthenticator(IPlugin):
     def cleanup(achalls):
         """Revert changes and shutdown after challenges complete.
 
+        This method should be able to revert all changes made by
+        perform, even if perform exited abnormally.
+
         :param list achalls: Non-empty (guaranteed) list of
             :class:`~certbot.achallenges.AnnotatedChallenge`
             instances, a subset of those previously passed to :func:`perform`.
@@ -304,8 +307,11 @@ class IInstaller(IPlugin):
 
         Both title and temporary are needed because a save may be
         intended to be permanent, but the save is not ready to be a full
-        checkpoint. If an exception is raised, it is assumed a new
-        checkpoint was not created.
+        checkpoint.
+
+        It is assumed that at most one checkpoint is finalized by this
+        method. Additionally, if an exception is raised, it is assumed a
+        new checkpoint was not finalized.
 
         :param str title: The title of the save. If a title is given, the
             configuration will be saved as a new checkpoint and put in a