Document timescaledb.bgw_scheduler_restart_time (#2748)

[mkindahl]

Description

[Short summary of why you created this PR]

Links

Fixes #[insert issue link, if any]

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Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• [ ] Is the content technically accurate?
• [ ] Is the content complete?
• [ ] Is the content presented in a logical order?
• [ ] Does the content use appropriate names for features and products?
• [ ] Does the content provide relevant links to further information?

Documentation team review checklist

• [ ] Is the content free from typos?
• [ ] Does the content use plain English?
• [ ] Does the content contain clear sections for concepts, tasks, and references?
• [ ] Have any images been uploaded to the correct location, and are resolvable?
• [ ] If the page index was updated, are redirects required and have they been implemented?
• [ ] Have you checked the built version of this content?