Display and delete invalid background jobs

[phil-davis]

WHAT Needs to be Documented?

New background:queue:status --display-invalid-jobs option.

This will be released in core 10.13.0 - so the doc changes do not need to be backpored to older doc branches.

WHERE Does This Need To Be Documented (Link)?

See PR #1066

WHY Should This Change Be Made?

core PR owncloud/core#40846 added the option.

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other