(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).
Could I convince you to use triple tildes for code blocks instead of triple backticks? It's less-nonstandard and will work on the docs site today.