Add docs and regression test about sorting the output of a recursive query in

Add docs and regression test about sorting the output of a recursive query in
depth-first search order.  Upon close reading of SQL:2008, it seems that the
spec's SEARCH DEPTH FIRST and SEARCH BREADTH FIRST options do not actually
guarantee any particular result order: what they do is provide a constructed
column that the user can then sort on in the outer query.  So this is actually
just as much functionality ...