(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 agree that adding a bit more detail about use case would be good, don't know if we want to get too specific about the type/provider stuff, though, since technically that stuff is probably considered to be a bug.

I might say something like:

`This can be useful if you are using multiple environments and want to make sure that your JRuby instances are cleaned up and don't have conflicts based on common code that appears in multiple environments.`

That's probably not ideal, either though... @nfagerlund @ahpook do you guys have any suggestions?

I might also consider throwing the word 'experimental' in here for this release, until we've had time to do more testing related to memory usage.