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)