Clarify ways for modifying config.toml when using GitLab-manager Runner

What does this MR do?

To me, it wasn't clear whether I can modify config.toml when using 1-click-install GitLab Runner within k8s. This MR intends to clarify that.

Here is the relevant Slack thread: https://gitlab.slack.com/archives/C3UCHUA76/p1602537009119900

Related issues

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When applicable:

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