pgsql: Allow plpgsql functions to omit RETURN command when the function

Log Message:
-----------
Allow plpgsql functions to omit RETURN command when the function returns
output parameters or VOID or a set. There seems no particular reason to
insist on a RETURN in these cases, since the function return value is
determined by other elements anyway. Per recent discussion.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.64 -> r1.65)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.64&r2=1.65)
pgsql/src/pl/plpgsql/src:
gram.y (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.68&r2=1.69)
pl_comp.c (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c.diff?r1=1.86&r2=1.87)
pl_exec.c (r1.134 -> r1.135)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.134&r2=1.135)
pgsql/src/test/regress/expected:
plpgsql.out (r1.27 -> r1.28)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.27&r2=1.28)
pgsql/src/test/regress/sql:
plpgsql.sql (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.22&r2=1.23)