pgsql: Update createuser examples to match the current program behavior,

Log Message:
-----------
Update createuser examples to match the current program behavior,
and add an example showing assignment of a password. Per suggestion
from Jari Aalto (via Martin Pitt).

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
createuser.sgml (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/createuser.sgml.diff?r1=1.39&r2=1.40)