Needs discussion of pg_xlog

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.4/static/runtime-config-file-locations.html
Description:

In various places (e.g. stackoverflow), I've seen people suggest having
pg_xlog ion a different disk (i.e. not in /var/lib/pgsql/9.X/data).

The only mention of this that I've seen is in Section 29.5 (WAL Internals),
and that just says "it is advantageous...", with no explanation.

If this IS a good idea, then "Server Configuration / File Locations" is
where it should be discussed: in particular, why is it a good idea, and how
does it affect backups (and in particular base backups to a standby server).