Added and corrected comments

category.go

@@ -14,6 +14,7 @@ type Category struct {
 	Label   string  `xml:"label,attr,omitempty"`
 }
 
+// NewCategory creates a new Category. It returns a *Category and an error.
 func NewCategory(term string) (*Category, error) {
 	content, err := NewContent(InlineText, "", "")
 	if err != nil {
@@ -23,10 +24,13 @@ func NewCategory(term string) (*Category, error) {
 	return &Category{Term: term, Content: content}, nil
 }
 
+// SetLabel sets the label of the Category.
 func (c *Category) SetLabel(label string) {
 	c.Label = html.UnescapeString(label)
 }
 
+// Check checks the Category for incompatibilities with RFC4287. It returns an
+// error.
 func (c *Category) Check() error {
 	if c.Term == "" {
 		return errors.New("term attribute of category empty")