Add doc section for force_checkpoint command

[aaron-seo]

Goes hand-in-hand with rabbitmq/rabbitmq-server#13175.

Adds documentation for rabbitmq-queues force_checkpoint from that PR.

[michaelklishin]

This file is generated from man pages in rabbitmq/rabbitmq-server.

So rabbitmq-queues.8 must be modified in that repo, then the HTML files should be regenerated and put into this repo, as the README linked to above explains.