P3914R0: Re NB US 228-348 (C++26 CD)

Fixes NB US 228-348 (C++26 CD).

Author: eisenwave

source/exec.tex

@@ -5340,8 +5340,10 @@
 
 \pnum
 Let \tcode{\placeholder{ssource-t}} be an unspecified type
-that models \exposconcept{stoppable-source} and
-let \tcode{ssource} be an lvalue of type \tcode{\placeholder{ssource-t}}.
+that models \exposconcept{stoppable-source} and \libconcept{default_initializable},
+such that a default-initialized object of type \tcode{\placeholder{ssource-t}}
+has an associated stop state.
+Let \tcode{ssource} be an lvalue of type \tcode{\placeholder{ssource-t}}.
 Let \tcode{\placeholder{stoken-t}} be \tcode{decltype(ssource.get_token())}.
 Let \exposid{future-spawned-sender} be the alias template: