Doc: Clean up references to obsolete OS versions.

Remove obsolete instructions for old operating system versions, and
update the text to reflect the defaults on modern systems.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/CA%2BhUKGLmJUSwybaPQv39rB8ABpqJq84im2UjZvyUY4feYhpWMw%40mail.gmail.com