Improve camouflage wording + screenshot (easy)

First of all, it seemed to be confirmed that the Windows camouflage will be removed from Tails Jessie (January 26). Still, if we get it back somehow, the way it is presented in the Greeter should make it more clear that this is about the visual appearance only (and not the networking behavior for example).

Parent Task: #10812 (closed)

Original created by @alant on 10822 (Redmine)