(SERVER-2822) Explain JRuby metrics in multithreaded mode
(SERVER-2822) Explain JRuby metrics in multithreaded mode

In MT mode, all the metrics that would normally refer to JRuby instances
instead refer to JRuby threads. This commit adds a couple of notes
explaining that.

puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server