mention /etc/salt/minion_id in docs explicitly (#32573)
mention /etc/salt/minion_id in docs explicitly (#32573)

This file is essential if one needs to change a minion's ID. Showing the
full path to the file makes it easier searching for it, e.g. Google::

    http://blog.mindfab.net/2013/11/salt-changing-minions-hostname.html
saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt