(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.
The wording here threw me off a bit. Because you start with "Puppet Server does not read this setting" and then continue with "Instead, this is now done through a service listed in the `bootstrap.cfg` file", it makes it sound like the setting IS read, it's just read by the CA service, whereas the setting is actually ignored by Puppet Server and the behavior is entirely determined by which version of the CA service you enable. This becomes more clear when following the link, but I think it's confusing here.

I think it would be best to just straight-off say that this setting is not honored by Puppet Server. Also, I'd more explicitly state that the behavior controlled by this setting is now controlled by enabling different services in your `bootstrap.cfg` file.