bgpv1: Add the documentation for using serviceAdvertisements

Ideally, the internalTrafficPolicy and externalTrafficPolicy fields are independent. However, since we advertise the ClusterIP to the external world via BGP, it has caused some confusion regarding the meanings of external and internal. Therefore, we need to provide some clarification in the documentation.

For ClusterIP: Only consider the iTP configuration for advertisement, and ignore the eTP configuration.
For ExternalIP and LBIP: Only consider the eTP configuration for advertisement, and ignore the iTP configuration.

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

Documentation/network/bgp-control-plane.rst

@@ -305,10 +305,15 @@ defaults to the ``LoadBalancerIP`` service. You can also specify the ``.serviceA
 field to advertise specific service types, with options such as ``LoadBalancerIP``,
 ``ClusterIP`` and ``ExternalIP``.
 
+It is worth noting that when you configure ``virtualRouters[*].serviceAdvertisements`` as ``ClusterIP``,
+the BGP Control Plane only considers the configuration of the service's ``.spec.internalTrafficPolicy`` and ignores
+the configuration of ``.spec.externalTrafficPolicy``.
+For ``ExternalIP`` and ``LoadBalancerIP``, it only considers the configuration of
+the service's ``.spec.externalTrafficPolicy`` and ignores the configuration of ``.spec.internalTrafficPolicy``.
+
 The ``.serviceSelector`` field is a label selector that selects Services matching
 the specified ``.matchLabels`` or ``.matchExpressions``.
 
-
 When your upstream router supports Equal Cost Multi Path(ECMP), you can use
 this feature to load balance traffic to the Service across multiple nodes by
 advertising the same ingress IPs from multiple nodes.