nixos/docker: change misleading error message

The socketActivation option was removed, but later on socket activation
was added back without the option to disable it. The description now reflects
that socket activation is used unconditionally in the current setup.

nixos/modules/virtualisation/docker.nix

@@ -216,7 +216,7 @@ in
   ]);
 
   imports = [
-    (mkRemovedOptionModule ["virtualisation" "docker" "socketActivation"] "This option was removed in favor of starting docker at boot")
+    (mkRemovedOptionModule ["virtualisation" "docker" "socketActivation"] "This option was removed and socket activation is now always active")
   ];
 
 }