update FAQ docs

custom vhosts must reside as the master vhost level to avoid GHOST vhost
detection -- /var/aegir/config/server_master/nginx/vhost.d
Merge pull request #361 from pricejn2/custom-vhost

update FAQ docs
Update and improve docs/FAQ.txt
Commons 2.20
Commons 2.20 has changed its profile name from drupal_commons to commons.
Do not rename the legacy Commons profile name.
Do not rename Commons 3.
Avoid "Is a directory" noise in the log.
Latest hotfix added on: Tue Aug 12 14:05:57 PDT 2014
Merge branch 'refs/heads/master' into 2.2.x-legacy