[skip ci]Add comment for ListNode (#13346)

Signed-off-by: NCai.Zhang <cai.zhang@zilliz.com>

internal/indexcoord/node_manager.go

@@ -124,6 +124,7 @@ func (nm *NodeManager) PeekClient(meta Meta) (UniqueID, types.IndexNode) {
 	return nodeID, client
 }
 
+// ListNode list all IndexNodes in node manager.
 func (nm *NodeManager) ListNode() []UniqueID {
 	nm.lock.Lock()
 	defer nm.lock.Unlock()