documentation: Remove list association from ATK_ROLE_DESCRIPTION_{TERM,VALUE}

There is a need for term-value pair roles outside of the context of an
ATK_ROLE_DESCRIPTION_LIST. Rather than create yet more roles, we can just
update the documentation so that the list association is not made within
the definition of ATK_ROLE_DESCRIPTION_{TERM,VALUE}.

https://bugzilla.gnome.org/show_bug.cgi?id=791020