SF patch #1379023: document that a weakref callback is not invoked if the
weakref object itself is deleted first.
diff --git a/Doc/lib/libweakref.tex b/Doc/lib/libweakref.tex
index 2f8ade0..840b674 100644
--- a/Doc/lib/libweakref.tex
+++ b/Doc/lib/libweakref.tex
@@ -75,8 +75,8 @@
retrieved by calling the reference object if the referent is still
alive; if the referent is no longer alive, calling the reference
object will cause \constant{None} to be returned. If \var{callback} is
- provided and not \constant{None},
- it will be called when the object is about to be
+ provided and not \constant{None}, and the returned weakref object is
+ still alive, the callback will be called when the object is about to be
finalized; the weak reference object will be passed as the only
parameter to the callback; the referent will no longer be available.