pgsql: Add docs and regression test about sorting the output of a

Log Message:
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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 ...

Modified Files:
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pgsql/doc/src/sgml:
queries.sgml (r1.49 -> r1.50)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/queries.sgml?r1=1.49&r2=1.50)
pgsql/src/test/regress/expected:
with.out (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/with.out?r1=1.7&r2=1.8)
pgsql/src/test/regress/sql:
with.sql (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/with.sql?r1=1.7&r2=1.8)