[execution.syn] Add comments to cross-references in synopsis

Fixes NB US 200-323 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -674,7 +674,7 @@
 
   struct dependent_sender_error : exception {};
 
-  // \ref{exec.getcomplsigs}
+  // \ref{exec.getcomplsigs}, get completion signatures
   template<class Sndr, class... Env>
     consteval auto get_completion_signatures() -> @\exposconcept{valid-completion-signatures}@ auto;
 
@@ -700,22 +700,22 @@
   struct @\libglobal{spawn_t}@ { @\unspec@ };
   inline constexpr spawn_t spawn{};
 
-  // \ref{exec.as.awaitable}
+  // \ref{exec.as.awaitable}, coroutine utility \tcode{as_awaitable}
   struct @\libglobal{as_awaitable_t}@ { @\unspec@ };
   inline constexpr as_awaitable_t @\libglobal{as_awaitable}@{};
 
-  // \ref{exec.with.awaitable.senders}
+  // \ref{exec.with.awaitable.senders}, coroutine utility \tcode{with_awaitable_senders}
   template<@\exposconcept{class-type}@ Promise>
     struct with_awaitable_senders;
 
   // \ref{exec.scope.concepts}, scope concepts
   template<class Token>
     concept @\libconcept{scope_token}@ = @\seebelow@;
 
-  // \ref{exec.scope.simple.counting}
+  // \ref{exec.scope.simple.counting}, simple counting scope
   class simple_counting_scope;
 
-  // \ref{exec.scope.counting}
+  // \ref{exec.scope.counting}, counting scope
   class counting_scope;
 }
 
@@ -735,14 +735,14 @@
 }
 
 namespace std::execution {
-  // \ref{exec.affine.on}
+  // \ref{exec.affine.on}, coroutine utility \tcode{affine_on}
   struct @\libglobal{affine_on_t}@ { @\unspec@ };
   inline constexpr affine_on_t @\libglobal{affine_on}@{};
 
-  // \ref{exec.inline.scheduler}
+  // \ref{exec.inline.scheduler}, inline scheduler
   class @\libglobal{inline_scheduler}@;
 
-  // \ref{exec.task.scheduler}
+  // \ref{exec.task.scheduler}, task scheduler
   class @\libglobal{task_scheduler}@;
 
   template<class E>
@@ -761,7 +761,7 @@
   template<@\libconcept{scheduler}@ Sch>
     change_coroutine_scheduler(Sch) -> change_coroutine_scheduler<Sch>;
 
-  // \ref{exec.task}
+  // \ref{exec.task}, class template \tcode{task}
   template<class T, class Environment>
     class @\libglobal{task}@;
 }