Improve documentation of the CREATEROLE attibute.

Improve documentation of the CREATEROLE attibute.

In user-manag.sgml, document precisely what privileges are conveyed
by CREATEROLE. Make particular note of the fact that it allows
changing passwords and granting access to high-privilege roles.
Also remove the suggestion of using a user with CREATEROLE and
CREATEDB instead of a superuser, as there is no real security
advantage to this approach.

Elsewhere in the documentation, adjust text that suggests that
CREATEROLE only allows for role creation, and
refer to the documentation in user-manag.sgml as appropriate.

Patch by me, reviewed by Álvaro Herrera

Discussion: http://postgr.es/m/CA+TgmoZBsPL8nPhvYecx7iGo5qpDRqa9k_AcaW1SbOjugAY1Ag@mail.gmail.com