Add instructions to pause Batched Background Migrations on GL 14.x

What does this MR do?

I had a conversation with @lbot today about a customer emergency last week that led to some possible changes to the Batched Background Migrations documentation, specific to 14.x. There is a Slack thread containing most of the details.

It's possible for administrators to pause Batched Background Migrations, for instance if they are seeing unusually high load on their hardware leading to performance issues, or for whatever other reason an administrator might want to do so. This MR adds instructions for doing so on GitLab versions 14.x, which were vetted and tested during the aforementioned customer emergency.

The condensed steps are:

1. Use the existing disable command in the ruby console.
2. Use the included database queries in the database console/prompt to confirm the migration has paused (i.e. no new batch created in the database).
3. Use the existing enable command in the ruby console when ready to continue with the migration.

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