[basic.life] Add cross-reference for union member lifetime

Author: jensmaurer

source/basic.tex

@@ -3358,7 +3358,8 @@
 except that if the object is a union member or subobject thereof,
 its lifetime only begins if that union member is the
 initialized member in the union\iref{dcl.init.aggr,class.base.init},
-or as described in \ref{class.union} and \ref{class.copy.ctor},
+or as described in
+\ref{class.union}, \ref{class.copy.ctor}, and \ref{class.copy.assign},
 and except as described in \ref{allocator.members}.
 The lifetime of an object \placeholder{o} of type \tcode{T} ends when:
 \begin{itemize}