Leave a few manual pages for post 2.0.0-pre.4

In particular the cowboy_sub_protocol is subject to change soon.
Also the cowboy_stream behavior is going to be left hidden in
this release and enabled when it becomes easier to chain them.

doc/src/manual/cowboy_app.asciidoc

@@ -41,8 +41,8 @@ Behaviors:
 * link:man:cowboy_loop(3)[cowboy_loop(3)] - Loop handlers
 * link:man:cowboy_middleware(3)[cowboy_middleware(3)] - Middlewares
 * link:man:cowboy_rest(3)[cowboy_rest(3)] - REST handlers
-* link:man:cowboy_stream(3)[cowboy_stream(3)] - Stream handlers
-* link:man:cowboy_sub_protocol(3)[cowboy_sub_protocol(3)] - Sub protocols
+// @todo * link:man:cowboy_stream(3)[cowboy_stream(3)] - Stream handlers
+// @todo * link:man:cowboy_sub_protocol(3)[cowboy_sub_protocol(3)] - Sub protocols
 * link:man:cowboy_websocket(3)[cowboy_websocket(3)] - Websocket handlers
 
 Middlewares:
@@ -50,6 +50,8 @@ Middlewares:
 * link:man:cowboy_router(3)[cowboy_router(3)] - Router middleware
 * link:man:cowboy_handler(3)[cowboy_handler(3)] - Handler middleware
 
+// @todo http_status_codes is not linked to; what to do with it?
+
 == Dependencies
 
 * link:man:ranch(7)[ranch(7)] - Socket acceptor pool for TCP protocols