More documentation update for GRANT ... WITH SET OPTION.

More documentation update for GRANT ... WITH SET OPTION.

Update the reference pages for various ALTER commands that
mentioned that you must be a member of role that will be the
new owner to instead say that you must be able to SET ROLE
to the new owner. Update ddl.sgml's generate statement on this
topic along similar lines.

Likewise, update CREATE SCHEMA and CREATE DATABASE, which
have options to specify who will own the new objects, to say
that you must be able to SET ROLE to the role that will own
them.

Finally, update the documentation for the GRANT statement
itself with some general principles about how the SET option
works and how it can be used.

Patch by me, reviewed (but not fully endorsed) by Noah Misch.

Discussion: http://postgr.es/m/CA+TgmoZk6VB3DQ83+DO5P_HP=M9PQAh1yj-KgeV30uKefVaWDg@mail.gmail.com