[Serve] Documentation and typing fixes for `serve.run(..., route_pref…

…ix=False)` (#44911)

In #44809 I thought that route_prefix didn't support being set to None to disable HTTP ingress because the type was str, and the docs didn't mention that this was supported (except indirectly in other places which I didn't know about until searching for them as part of this PR). It turns out this feature is already well-supported throughout the controller and proxy, it just wasn't fully documented in the public API, so this PR fixes that.

---------

Signed-off-by: Josh Karpel <[email protected]>
Co-authored-by: Edward Oakes <[email protected]>

python/ray/serve/api.py

@@ -546,7 +546,7 @@ def run(
     target: Application,
     blocking: bool = False,
     name: str = SERVE_DEFAULT_APP_NAME,
-    route_prefix: str = DEFAULT.VALUE,
+    route_prefix: Optional[str] = DEFAULT.VALUE,
     logging_config: Optional[Union[Dict, LoggingConfig]] = None,
 ) -> DeploymentHandle:
     """Run an application and return a handle to its ingress deployment.
@@ -565,9 +565,10 @@ def run(
             will loop and log status until Ctrl-C'd.
         name: Application name. If not provided, this will be the only
             application running on the cluster (it will delete all others).
-        route_prefix: Route prefix for HTTP requests. If not provided, it will use
-            route_prefix of the ingress deployment. If specified neither as an argument
-            nor in the ingress deployment, the route prefix will default to '/'.
+        route_prefix: Route prefix for HTTP requests. Defaults to '/'.
+            If `None` is passed, the application will not be exposed over HTTP
+            (this may be useful if you only want the application to be exposed via
+            gRPC or a `DeploymentHandle`).
         logging_config: Application logging config. If provided, the config will
             be applied to all deployments which doesn't have logging config.