Update service-defaults.mdx (#8780)

website/pages/docs/agent/config-entries/service-defaults.mdx

@@ -37,7 +37,7 @@ Protocol  = "http"
   by Connect proxies for things like observability features and to unlock usage
   of the [`service-splitter`](/docs/agent/config-entries/service-splitter) and
   [`service-router`](/docs/agent/config-entries/service-router) config
-  entries for a service.
+  entries for a service. Supported values are one of `tcp`, `http`, `http2`, or `grpc`.
 
 - `MeshGateway` `(MeshGatewayConfig: <optional>)` - Controls the default
   [mesh gateway configuration](/docs/connect/mesh-gateway#connect-proxy-configuration)