Clarified Add method's comments

commonAttributes.go

@@ -17,7 +17,8 @@ func newCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddAttribute adds the attribute to the CommonAttributes. It returns an int.
+// AddAttribute adds the attribute to the CommonAttributes. It returns its index
+// as an int.
 func (c *CommonAttributes) AddAttribute(name, value string) int {
 	return addToSlice(&c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
 }