Clarified Add method's comments

person.go

@@ -21,7 +21,8 @@ func NewPerson(name string) *Person {
 	}
 }
 
-// AddExtension adds the Extension to the Person. It returns an int.
+// AddExtension adds the Extension to the Person. It returns its index as an
+// int.
 func (p *Person) AddExtension(e *ExtensionElement) int {
 	return addToSlice(&p.Extensions, e)
 }