I added the following missing comment block just before "namespace Folks":

=======================================================
types.vala:

/**
 * New general types required by Folks.
 *
 * @since 0.3.1
 */
namespace Folks
{
  /**
   * A 'boolean' type that has a distinct 'unset' state.
   *
   * @since 0.3.1
   */
  public enum MaybeBool
    {
...
=======================================================

However, valadoc ignores it since it isn't directly above a class. So my gtk-doc documentation index has a link for "types" (without any description, as the classes have).

"types" doesn't show up at all in the native doclet page (it just links MaybeBool directly from the index).

MaybeBool's docs show up as expected in the page for "types".