Update puma signals

This updates documentation how to perform almost zero-downtime deployments.

Puma is already properly restarted as part of gitlab-ctl reconfigure:

runit_service svc do
  down node['gitlab'][svc]['ha']
  restart_command "2" # Restart Puma using SIGUSR2
  template_name 'puma'
  control ['t']

Closes #4739 (closed)