[1.6.x] Fixed #21296 -- Added docs for PostgreSQL and unix domains sockets.

If you're using PostgreSQL, by default (empty :setting:`HOST`), the connection

to the database is done through UNIX domain sockets ('local' lines in

``pg_hba.conf``). If you want to connect through TCP sockets, set

:setting:`HOST` to 'localhost' or '127.0.0.1' ('host' lines in ``pg_hba.conf``).

``pg_hba.conf``). If your UNIX domain socket is not in the standard location,

use the same value of ``unix_socket_directory`` from ``postgresql.conf``.

If you want to connect through TCP sockets, set :setting:`HOST` to 'localhost'

or '127.0.0.1' ('host' lines in ``pg_hba.conf``).

On Windows, you should always define :setting:`HOST`, as UNIX domain sockets

are not available.