Add comment to alternateRelExists check

entry.go

@@ -81,6 +81,9 @@ func (e *Entry) Check() error {
 			return fmt.Errorf("content element of entry %v: %v", e.ID.URI, err)
 		}
 	} else {
+		// atom:entry elements that contain no child atom:content element MUST
+		// contain at least one atom:link element with a rel attribute value of
+		// "alternate".
 		if !alternateRelExists(e.Links) {
 			return errors.New("no content element of entry %v and no link element with rel \"alternate\"")
 		}