Added and corrected comments

plainText.go

@@ -8,8 +8,11 @@ type PlainText struct {
 	Text string `xml:"text"`
 }
 
+// isText checks whether the PlainText is a Text. It returns a bool.
 func (p *PlainText) isText() bool { return true }
 
+// Check checks the PlainText for incompatibilities with RFC4287. It returns an
+// error.
 func (p *PlainText) Check() error {
 	if p.Type != "" && p.Type != "text" && p.Type != "html" {
 		return errors.New("type attribute of plain text must be text or html if not omitted")