Update secondary-network docs (#6875)

Signed-off-by: Lan Luo <[email protected]>

docs/feature-gates.md

@@ -329,9 +329,7 @@ This feature is only supported:
 ### SecondaryNetwork
 
 The `SecondaryNetwork` feature enables support for provisioning secondary network interfaces for Pods, by annotating
-them appropriately.
-
-More documentation will be coming in the future.
+them appropriately. Refer to this [document](secondary-network.md) for more information
 
 #### Requirements for this Feature
 

docs/secondary-network.md

@@ -68,10 +68,14 @@ data:
 ```
 
 At the moment, Antrea supports only a single OVS bridge for secondary networks,
-and supports up to eight physical interfaces on the bridge. The physical
-interfaces cannot be the Node's management interface, otherwise the Node's
-management network connectivity can be broken after `antrea-agent` creates the
-OVS bridge and moves the management interface to the bridge.
+and supports up to eight physical interfaces on the bridge.
+
+Note: when you set the Node's primary NIC as a secondary bridge physical interface,
+if the Node IP is assigned via DHCP and the DNS server is auto-configured by a DNS
+manager (e.g. system-resolved), you may lose the DNS configuration after the interface
+is moved to the OVS bridge, because of the interface state change. Please consider providing
+a static DNS configuration in `/etc/systemd/resolved.conf` before installing Antrea to
+use the primary NIC as a physical interface. Check more details on [issue 6558](https://github.com/antrea-io/antrea/issues/6558).
 
 ## Secondary VLAN network configuration
 

pkg/agent/secondarynetwork/init_linux.go

@@ -40,7 +40,6 @@ func (c *Controller) Initialize() error {
 		phyInterfaces := make([]string, len(c.secNetConfig.OVSBridges[0].PhysicalInterfaces))
 		copy(phyInterfaces, c.secNetConfig.OVSBridges[0].PhysicalInterfaces)
 		if len(phyInterfaces) == 1 {
-
 			bridgedName, _, err := util.PrepareHostInterfaceConnection(
 				c.ovsBridgeClient,
 				phyInterfaces[0],