Document runner long polling configuration

What does this MR do and why?

By default, long polling for runners is not enabled, but it can be beneficial to enable this on GitLab instances with many runners. This commit documents the benefits and the settings for long polling to work.

Relates to #323105 (closed)

Review App: https://main-ee-146701.docs.gitlab-review.app/ee/ci/runners/long_polling.html