[optional.ref.assign] Add missing Returns element

Author: eisenwave

source/utilities.tex

@@ -4748,6 +4748,10 @@
 \pnum
 \effects
 Equivalent to: \tcode{\exposid{convert-ref-init-val}(std::forward<U>(u))}.
+
+\pnum
+\returns
+\tcode{*\exposid{val}}.
 \end{itemdescr}
 
 \rSec3[optional.ref.swap]{Swap}