Added and corrected comments

extensionElement.go

@@ -10,10 +10,14 @@ type ExtensionElement struct {
 	XMLName xml.Name
 }
 
+// NewExtensionElement creates a new ExtensionElement. It returns a
+// *ExtensionElement.
 func NewExtensionElement(name string, value any) *ExtensionElement {
 	return &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value}
 }
 
+// Check checks the ExtensionElement for incompatibilities with RFC4287. It
+// returns an error.
 func (e *ExtensionElement) Check() error {
 	if e.Value == nil {
 		return errors.New("value element of extension element empty")