Add route port matching doc

Author: frankbu

site-src/api-types/httproute.md

@@ -43,6 +43,64 @@ spec:
 Note that the target Gateway needs to allow HTTPRoutes from the route's
 namespace to be attached for the attachment to be successful.
 
+You can also attach routes to specific sections of the parent resource.
+For example, let's say that the `acme-lb` Gateway includes the following
+listeners:
+
+```yaml
+  listeners:
+  - name: foo
+    protocol: HTTP
+    port: 8080
+    ...
+  - name: bar
+    protocol: HTTP
+    port: 8090
+    ...
+  - name: baz
+    protocol: HTTP
+    port: 8090
+    ...
+```
+
+You can bind a route to listener `foo` only, using the `sectionName` field
+in `parentRefs`:
+
+```yaml
+spec:
+  parentRefs:
+  - name: acme-lb
+    sectionName: foo
+```
+
+Alternatively, you can achieve the same effect by using the `port` field,
+instead of `sectionName`, in the `parentRefs`:
+
+```yaml
+spec:
+  parentRefs:
+  - name: acme-lb
+    port: 8080
+```
+
+Binding to a port also llows you to attach to multiple listeners at once.
+For example, binding to port `8090` of the `acme-lb` Gateway would be more
+convenient than binding to the corresponding listeners by name:
+
+```yaml
+spec:
+  parentRefs:
+  - name: acme-lb
+    sectionName: bar
+  - name: acme-lb
+    sectionName: baz
+```
+
+However, when binding Routes by port number, Gateway admins will no longer have
+the flexibility to switch ports on the Gateway without also updating the Routes.
+The approach should only be used when a Route should apply to a specific port
+number as opposed to listeners whose ports may be changed.
+
 ### Hostnames
 
 Hostnames define a list of hostnames to match against the Host header of the

site-src/guides/tcp.md

@@ -49,8 +49,24 @@ In this way each `TCPRoute` "attaches" itself to a different port on the
 `Gateway` so that the service `my-foo-service` is taking traffic for port `8080`
 from outside the cluster and `my-bar-service` takes the port `8090` traffic.
 
+Note that you can achieve this same result by binding the Routes to the Gateway
+listeners using the `port` field in the `parentRefs`:
+
+```yaml
+spec:
+  parentRefs:
+  - name: my-tcp-gateway
+    port: 8080
+```
+
+Using the `port` field instead of `sectionName` for the attachment has the
+downside of more tightly coupling the relationship between the Gateway and
+its associated Routes. Refer to [Attaching to Gateways][attaching] for more
+details.
+
 [tcproute]:/reference/spec/#gateway.networking.k8s.io/v1alpha2.TCPRoute
 [tcp]:https://datatracker.ietf.org/doc/html/rfc793
 [httproute]:/reference/spec/#gateway.networking.k8s.io/v1alpha2.HTTPRoute
 [gateway]:/reference/spec/#gateway.networking.k8s.io/v1alpha2.Gateway
 [svc]:https://kubernetes.io/docs/concepts/services-networking/service/
+[attaching]:/api-types/httproute/#attaching-to-gateways