api_docs: Prep work to document zilencer endpoints.

Until now we were not documenting bouncer's REST API endpoints.

We plan to document the newly introduced "remotes/push/e2ee/register"
and "remotes/push/e2ee/notify" endpoints.

This commit does the prep work for documenting bouncer endpoints:
* mark the older endpoints related to sending non-E2EE push
  notifications as "intentionally_undocumented" - we'll remove
  them in future.
* the remaining endpoints are marked pending-to-document with
  helpful comments.

zilencer/auth.py

@@ -172,6 +172,7 @@ def remote_server_dispatch(request: HttpRequest, /, **kwargs: Any) -> HttpRespon
 
 def remote_server_path(
     route: str,
-    **handlers: Callable[Concatenate[HttpRequest, RemoteZulipServer, ParamT], HttpResponse],
+    **handlers: Callable[Concatenate[HttpRequest, RemoteZulipServer, ParamT], HttpResponse]
+    | tuple[Callable[Concatenate[HttpRequest, RemoteZulipServer, ParamT], HttpResponse], set[str]],
 ) -> URLPattern:
     return path(route, remote_server_dispatch, handlers)