Document app protocol (#7052)

* Document app protocol

* Fix lint

content/en/docs/ops/configuration/traffic-management/protocol-selection/index.md

@@ -16,7 +16,13 @@ This can be done automatically or explicitly specified.
 
 ## Manual protocol selection
 
-Protocols can be specified manually by naming the Service port `name: <protocol>[-<suffix>]`.
+Protocols can be specified manually in the Service definition.
+
+This can be configured in two ways:
+
+- By the name of port the port, like `name: <protocol>[-<suffix>]`.
+- In Kubernetes 1.18+, by the `appProtocol` field, like `appProtocol: <protocol>`.
+
 The following protocols are supported:
 
 - `grpc`
@@ -34,7 +40,7 @@ The following protocols are supported:
 \* These protocols are disabled by default to avoid accidentally enabling experimental features.
 To enable them, configure the corresponding Pilot [environment variables](/docs/reference/commands/pilot-discovery/#envvars).
 
-Below is an example of a Service that defines a `mysql` port and an `http` port:
+Below is an example of a Service that defines a `mysql` port by `appProtocol` and an `http` port by name:
 
 {{< text yaml >}}
 kind: Service
@@ -43,7 +49,8 @@ metadata:
 spec:
   ports:
   - number: 3306
-    name: mysql
+    name: database
+    appProtocol: mysql
   - number: 80
     name: http-web
 {{< /text >}}