rebuild an agent's closure when the binary cache lost it

nix/host-modules/swarm-attic.nix

@@ -166,7 +166,8 @@ in
       description = ''
         How long a store path stays in the cache after it was added or last
         fetched. A garbage collection every twelve hours removes what is older;
-        a build the cache no longer holds is rebuilt on the next miss.
+        swarm-controller rebuilds a closure the cache no longer holds and
+        the builder pushes it again.
       '';
     };
   };