Graceful shutdown documentation

This is a general Merge Request template. Consider to choose a template from the list above if it will match your case more.

What does this MR do?

Improves the documentation surrounding graceful shutdown, resolving #1793 (closed)

Why was this MR needed?

When I attempted a graceful shutdown on Ubuntu 14.04 I thought it hadn't worked. It was confusing.

Are there points in the code the reviewer needs to double check?

I don't personally use autoscaling, but I've updated the autoscaling documentation. There are some differences between my setup and what's documented - for example where the documentation says to use killall -SIGQUIT gitlab-runner I instead had to use sudo killall -SIGQUIT gitlab-ci-multi-runner - and I don't know if that's due to my setup or the documentation being outdated.

Does this MR meet the acceptance criteria?

• [✔] Documentation created/updated
• Tests
  • [✘] Added for this feature/bug
  • [✔] All builds are passing
• [✔] Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Resolves #1793 (closed)