manually update description in crd yaml for k8s 1.18

Author: kayx23

config/crd-nocel/apisix.apache.org_v2.yaml

@@ -1433,7 +1433,8 @@ spec:
                 type: object
               externalNodes:
                 description: |-
-                  ExternalNodes defines a static list of backend nodes located outside the cluster.
+                  ExternalNodes defines a static list of backend nodes. These can be external hosts
+                  outside the cluster or cluster-internal Services specified by their DNS name.
                   When this field is set, the upstream will route traffic directly to these nodes
                   without DNS resolution or service discovery.
                 items: