Document that values returned by 'sortKey' must be strings.

This requirement is needed to ensure a total ordering, e.g. in Py3k where
strings and ints are not comparable.

CHANGES.txt

@@ -4,7 +4,8 @@ Changes
 1.4.1 (unreleased)
 ------------------
 
-- TBD
+- Document that values returned by ``sortKey`` must be strings, in order
+  to guarantee total ordering.
 
 1.4.0 (2013-01-03)
 ------------------

transaction/interfaces.py

@@ -396,6 +396,8 @@ class IDataManager(zope.interface.Interface):
     def sortKey():
         """Return a key to use for ordering registered DataManagers.
 
+        In order to guarantee a total ordering, keys must be strings.
+
         ZODB uses a global sort order to prevent deadlock when it commits
         transactions involving multiple resource managers.  The resource
         manager must define a sortKey() method that provides a global ordering