Docs: Clarification on the background migration for upgrade instructions or upgrade barometer

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For background migrations we have not made it explicit to wait until it's completed before jump to a newer release. If we don't wait from 9.5.x to 10.x we'll likely see a partial events table. But for 11.3 to 11.4 upgrade, apparently it's not required as gitlab.com did not wait for it as we probably plan to do the switch over after 3 releases. See https://about.gitlab.com/2018/09/22/gitlab-11-3-released/#barometer

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

• Doc title

• Feature overview/description

• Feature use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template