Document encrypted websockets

Yeah, a few points:

1. Avoid the "starting with Foreman 1.6", it dates very quickly :)
2. As @stbenjam, people might not have the config.  I'd suggest explaining how to add it if you upgrade, or perhaps better, put it into the 3.6 Upgrade section and link to it.
3. Keep this about:config advice as a fallback for now I'd say.
4. Remember that only VNC connections are encrypted, libvirt SPICE ones won't be: http://projects.theforeman.org/issues/7018