Clarify minor point about the ref() and proxy() constructors.
This matches what is already documented for corresponding feature of the C API.
diff --git a/Doc/lib/libweakref.tex b/Doc/lib/libweakref.tex
index 12a1220..fc88916 100644
--- a/Doc/lib/libweakref.tex
+++ b/Doc/lib/libweakref.tex
@@ -58,7 +58,8 @@
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 \code{None} to be returned. If \var{callback} is
- provided, it will be called when the object is about to be
+ provided and not \constant{None},
+ it 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.