LWG4346 task::promise_type::return_void/value lack a specification

Fixes NB US 250-389 (C++26 CD).

Author: eisenwave

source/exec.tex

@@ -7625,6 +7625,27 @@
 \tcode{noop_coroutine()}.
 \end{itemdescr}
 
+\indexlibrarymember{return_void}{task::promise_type}%
+\begin{itemdecl}
+void return_void();
+\end{itemdecl}
+\begin{itemdescr}
+\pnum
+\effects
+Does nothing.
+\end{itemdescr}
+
+\indexlibrarymember{return_value}{task::promise_type}%
+\begin{itemdecl}
+template<class V>
+  void return_value(V&& v);
+\end{itemdecl}
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to \tcode{\exposid{result}.emplace(std::forward<V>(v))}.
+\end{itemdescr}
+
 \indexlibrarymember{get_env}{task::promise_type}%
 \begin{itemdecl}
 @\unspec@ get_env() const noexcept;