pgsql: Clarify documentation about PASSWORD in CREATE/ALTER ROLE

Clarify documentation about PASSWORD in CREATE/ALTER ROLE

The documentation of CREATE/ALTER ROLE has been missing two things
related to PASSWORD:
- The password value provided needs to be quoted, some places of the
documentation marked the field with quotes, but not others, which led to
confusion.
- PASSWORD NULL was not provided consistently, with ENCRYPTED being not
compatible with it.

Reported-by: Steven Winfield
Author: Michael Paquier
Reviewed-by: David G. Johnston
Discussion: https://postgr.es/m/154282901979.1316.7418475422120496802@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d392e9bdea957964e1fa6a5481e5adb5904d759a

Modified Files
--------------
doc/src/sgml/ref/alter_role.sgml  | 5 +++--
doc/src/sgml/ref/alter_user.sgml  | 2 +-
doc/src/sgml/ref/create_role.sgml | 5 +++--
doc/src/sgml/ref/create_user.sgml | 2 +-
4 files changed, 8 insertions(+), 6 deletions(-)