(SERVER-250) Document jruby-pool Admin endpoint

Document the new jruby-pool endpoint that was recently added
to the HTTP Admin API.

This should be merged after https://github.com/puppetlabs/puppet-server/pull/317 (or the PR that will replace it once https://github.com/puppetlabs/puppet-server/pull/313 is merged).
I also think it would be good to include some more details in here about why someone might want to use this.  For example, I believe if you have some Ruby Puppet code -- types/providers specifically -- which differs between environments that you could end up in a scenario where code for the "wrong" environment could be used during the catalog compilation process.  Invoking this endpoint would allow the latest Ruby code to be reloaded from disk.  Seems like this problem surfaces only with certain kinds of code differences, e.g., signature changes - not sure.  I think @cprice404 or @dankreek could provide a more concrete case that we could document here.