fixup: header exposconcept

Author: jensmaurer

source/exec.tex

@@ -572,7 +572,7 @@
   template<class Fn>
     concept @\exposconcept{completion-signature}@ = @\seebelow@;  // \expos
 
-  template<completion-signature... Fns>
+  template<@\exposconcept{completion-signature}@... Fns>
     struct completion_signatures {};
 
   template<class Sigs>
@@ -994,6 +994,12 @@
 
 \rSec1[exec.recv]{Receivers}
 
+\rSec2[exec.recv.concepts]{Receiver concepts}
+
+\pnum
+A receiver represents the continuation of an asynchronous operation.
+The \libconcept{receiver} concept defines the requirements for a receiver type ([async.ops]). The receiver_of concept defines the requirements for a receiver type that is usable as the first argument of a set of completion operations corresponding to a set of completion signatures. The get_env customization point object is used to access a receiver’s associated environment.
+
 \rSec1[exec.opstate]{Operation states}
 
 \rSec1[exec.snd]{Senders}