Update the documentation to reflect the changes to ReferenceError.

Doc/lib/libexcs.tex

@@ -237,6 +237,17 @@ Raised when an \keyword{assert} statement fails.
   typical applications prefer to drop bits than raise an exception.
 \end{excdesc}
 
+\begin{excdesc}{ReferenceError}
+  This exception is raised when a weak reference proxy, created by the
+  \function{\refmodule{weakref}.proxy()} function, is used to access
+  an attribute of the referent after it has been garbage collected.
+  For more information on weak references, see the \refmodule{weakref}
+  module.
+  \versionadded[Previously known as the
+                \exception{\refmodule{weakref}.ReferenceError}
+                exception]{2.2}
+\end{excdesc}
+
 \begin{excdesc}{RuntimeError}
   Raised when an error is detected that doesn't fall in any of the
   other categories.  The associated value is a string indicating what

Doc/lib/libweakref.tex

@@ -111,7 +111,8 @@ be made to support weak references; see section \ref{weakref-extension},
 
 \begin{excdesc}{ReferenceError}
   Exception raised when a proxy object is used but the underlying
-  object has been collected.
+  object has been collected.  This is the same as the standard
+  \exception{ReferenceError} exception.
 \end{excdesc}