Clarify / revise gitlab-runner SIGQUIT config

What does this MR do?

This MR aims to add clarity to our instructions on setting up gitlab-runner to perform graceful shutdowns via systemd.

Specifically:

1. Add a step within the gitlab-runner graceful stop guidance that advises to reload systemd so that the altered configuration files are applied.
2. Specify a valid TimeoutStopSec example. Previously this was set to __REDACTED__, presumably in reference to the config example being pulled from a real world example, and the true value being redacted. It would be better to simply provide a valid working example that systemd can work with, as some users are likely to copy the values out of this verbatim, and __REDACTED__ is not valid. I've simply chosen infinity in this case where this seems to match the overall intention of the guidance here.

Related issues

This MR is being raised in reaction to a SaaS Premium customer having reached out to the support team via a support ticket, after having faced some difficulties implementing the gitlab-runner graceful shutdown configuration per our documentation.

• Zendesk (internal use only)
• Salesforce (internal use only)

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