Update to the iptables doc to explain set_policy is for default rules. Also a modification to the FAQ.

Just a quick update here, resolves https://github.com/saltstack/salt/issues/8504 . As well as adding a section to https://github.com/saltstack/salt/issues/8462 that talks about running two different versions of Salt after 0.17.1. PLEASE NOTE that I made an assumption regarding the actions of the Salt team based on prior actions, if this information is incorrect, please modify it!