a11y: enhance documentation
For unexposed following API
- elm_atspi_attributes_list_free
- elm_atspi_relation_clone
- elm_atspi_relation_free
- elm_atspi_relation_set_clone
- elm_atspi_relation_set_free
- elm_atspi_relation_set_relation_append
- elm_atspi_relation_set_relation_remove
- elm_atspi_relation_set_relation_type_remove
- elm_atspi_text_text_attribute_free
- elm_atspi_bridge_connected_get
- elm_access_object_register
- elm_access_object_unregister
- elm_access_object_get
Change-Id: I1c20596243b319544a14983a48976055e7ac4186