(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 "delete [...] their memory from the pool" is not technically correct.  I'd reword this sentence to be "This will remove all of the existing JRuby interpreters from the pool, allowing the memory occupied by these interpreters to be reclaimed by the JVM's garbage collector", or something along those lines.