Comment reason for non-exact match in direct cname loop checks (#5294)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/backend_lookup.go

@@ -30,6 +30,7 @@ func A(ctx context.Context, b ServiceBackend, zone string, state request.Request
 		case dns.TypeCNAME:
 			if Name(state.Name()).Matches(dns.Fqdn(serv.Host)) {
 				// x CNAME x is a direct loop, don't add those
+				// in etcd/skydns w.x CNAME x is also direct loop due to the "recursive" nature of search results
 				continue
 			}
 
@@ -104,6 +105,7 @@ func AAAA(ctx context.Context, b ServiceBackend, zone string, state request.Requ
 			// Try to resolve as CNAME if it's not an IP, but only if we don't create loops.
 			if Name(state.Name()).Matches(dns.Fqdn(serv.Host)) {
 				// x CNAME x is a direct loop, don't add those
+				// in etcd/skydns w.x CNAME x is also direct loop due to the "recursive" nature of search results
 				continue
 			}
 
@@ -358,6 +360,7 @@ func TXT(ctx context.Context, b ServiceBackend, zone string, state request.Reque
 		case dns.TypeCNAME:
 			if Name(state.Name()).Matches(dns.Fqdn(serv.Host)) {
 				// x CNAME x is a direct loop, don't add those
+				// in etcd/skydns w.x CNAME x is also direct loop due to the "recursive" nature of search results
 				continue
 			}