Merge revs 28249 and 28250 from trunk.

Document _p_invalidate().

src/persistent/interfaces.py

@@ -149,7 +149,8 @@ class IPersistent(Interface):
           - call _p_deactivate()
           - set _p_changed to None
 
-    There is one way to invalidate an object:  delete its _p_changed
+    There are two ways to invalidate an object:  call the
+    _p_invalidate() method (preferred) or delete its _p_changed
     attribute.  This cannot be ignored, and is used when semantics
     require invalidation.  Normally, an invalidated object transitions
     to the ghost state.  However, some objects cannot be ghosts.  When
@@ -235,6 +236,15 @@ class IPersistent(Interface):
         may choose to keep an object in the saved state.
         """
 
+    def _p_invalidate():
+        """Invalidate the object.
+
+        Invalidate the object.  This causes any data to be thrown
+        away, even if the object is in the changed state.  The object
+        is moved to the ghost state; further accesses will cause
+        object data to be reloaded.
+        """
+
 class IPersistentNoReadConflicts(IPersistent):
     def _p_independent():
         """Hook for subclasses to prevent read conflict errors.