(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).
"flush" may be a bit ambiguous here.  Could say something like this instead?

> This will delete all of the JRuby instances and associated memory from the pool.  The pool will be refilled with new JRuby instances.  The new instances will load the latest Ruby code and related resources from disk.