Reorganize/clarify cluster documentation

@mglukhovsky - this adds a little preface and moves the configuration file description to the top of the document rather than the end; I think it's probably better to talk about configuration options first, because they're applicable whether you're using `init.d` or `systemd`. (We also should probably add something for OS X, but that's another ticket, and I don't know what defaults we actually expect under that...)