Clarify minor point about the ref() and proxy() constructors.

This matches what is already documented for corresponding feature of the C API.

Doc/lib/libweakref.tex

@@ -58,7 +58,8 @@ be made to support weak references; see section \ref{weakref-extension},
   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.