(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 think this is an okay disclaimer; however, I'd also add something along the lines of "This is a computationally-expensive operation, and Puppet Server will be unable to fulfill incoming requests until the first of the new interpreters has been initialized, which may take several seconds".