pgsql: Add documentation about running postmasters in FreeBSD jails (use

Log Message:
-----------
Add documentation about running postmasters in FreeBSD jails (use
separate users).

Modified Files:
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pgsql/doc/src/sgml:
runtime.sgml (r1.366 -> r1.367)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.366&r2=1.367)