minor xds comments (istio#37534)

* minor xds comments

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

* comment

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

* Update context.go

Co-authored-by: Zhonghu Xu <xuzhonghu@huawei.com>

pilot/pkg/model/context.go

@@ -317,6 +317,7 @@ type WatchedResource struct {
 	// ResourceNames tracks the list of resources that are actively watched.
 	// For LDS and CDS, all resources of the TypeUrl type are watched if it is empty.
 	// For endpoints the resource names will have list of clusters and for clusters it is empty.
+	// For Delta Xds, all resources of the TypeUrl that a client has subscribed to.
 	ResourceNames []string
 
 	// VersionSent is the version of the resource included in the last sent response.