Document support for --cpu-rt-period and --cpu-rt-runtime

### Problem description

Document support for --cpu-rt-period and --cpu-rt-runtime

See https://github.com/docker/docker/pull/23430#pullrequestreview-5960208

### Project version(s) affected

Engine 1.13

cc/ @thaJeztah @erikstmartin