Add comments to author checks

entry.go

@@ -27,6 +27,10 @@ type Entry struct {
 	Extensions   []*ExtensionElement `xml:",any,omitempty"`
 }
 
+// atom:entry elements MUST contain one or more atom:author elements, unless
+// the atom:entry contains an atom:source element that contains an atom:author
+// element or, in an Atom Feed Document, the atom:feed element contains an
+// atom:author element itself.
 func (e *Entry) checkAuthors() error {
 	if e.Authors == nil {
 		if e.Source.Authors == nil {
@@ -81,6 +85,9 @@ 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\"")
 		}

feed.go

@@ -142,6 +142,9 @@ func (f *Feed) Check() error {
 		}
 	}
 
+	// atom:feed elements MUST contain one or more atom:author elements, unless
+	// all of the atom:feed element's child atom:entry elements contain at
+	// least one atom:author element.
 	if f.Authors == nil {
 		for _, e := range f.Entries {
 			if err := e.checkAuthors(); err != nil {