pgsql: Code review for UPDATE SET (columnlist) patch.

Log Message:
-----------
Code review for UPDATE SET (columnlist) patch.  Make it handle as much
of the syntax as this fundamentally dead-end approach can, in particular
combinations of single and multi column assignments.  Improve rather
inadequate documentation and provide some regression tests.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        update.sgml (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/update.sgml.diff?r1=1.39&r2=1.40)
    pgsql/src/backend/parser:
        gram.y (r2.564 -> r2.565)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.564&r2=2.565)
    pgsql/src/test/regress/expected:
        update.out (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/update.out.diff?r1=1.4&r2=1.5)
    pgsql/src/test/regress/sql:
        update.sql (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/update.sql.diff?r1=1.4&r2=1.5)