Document possible values for machine runner log settings [ONPREM-585]

[christian-stephen]

Description

A brief description of the changes.

Document the possible values for the machine runner logging settings, along with some description. Previously, we only documented the default values.

Reasons

A link to a GitHub and/or JIRA issue (if applicable).
Otherwise, a brief sentence about why you made these changes.

https://circleci.atlassian.net/browse/ONPREM-585

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