(SERVER-138) Created puppet.conf differences doc

This commit introduces a new document which contains some information
about the differences in supported settings from the `puppet.conf` file
for Puppet Server as opposed to the Ruby-based Puppet master.
I still feel pretty strongly that these should at least be separated from the rest of the settings, because I still think that they are largely irrelevant and confusing.  But that's the last I'll say on the matter, and will defer to @ahpook and @nfagerlund .