(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'm open to whatever section these make sense to live in.  Several of the other settings on this document, though, are also meaningful in both an agent and a master context.  I think @cprice404 wanted to highlight these ones in particular, though, because users might be more inclined to think of them as being used in an agent context.