Add docs about how to finalize BBM

What does this MR do and why?

Add docs about how finalizing BBM

This adds a specific section that describes what is involved in finalizing a batched background migration. This information is somewhat scattered around examples and other places right now and it didn't seem like we had any specific place to put it.

The main reason I wanted to add this doc to begin with was to highlight that the gitlab_schema must not change from queue to ensure. But I couldn't find any place to put it so I made a new section and then I also dumped all the other rules I know of in this section.

The gitlab_schema rule documented here will become very important in the next few months as we'll be changing hundreds of tables from gitlab_main to gitlab_main_cell so we want to be extra clear about this.

This is also in response to this issue we had before !139353 (merged) . It would be nice if this was detected by RSpec but then ultimately I'm not really sure what would be the right strategy for that. Because clearly we want to support running the same migration with different gitlab_schema if we are persisting this in the DB. I guess at least we could require that any ensure_ corresponds to some queue_ that was added in the past. But then again our implementation is very forgiving and just allows us to ignore when it doesn't find anything so I don't really get the reasoning behind that.

Also see !139876 (merged) for a related fix to our migration helpers.

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