Don't require class documentation in background migrations anymore

What does this MR do and why?

Don't require class documentation in background migrations anymore

This Rubocop rule was added in b3331cf3 . It is ignored a lot but now that we have db/docs/batched_background_migrations this is also redundant because we already have to document the background migration in the YML file. We don't want to duplicate this effort so we can remove this rule.

$ git grep 'disable Style/Documentation' |wc -l
      65

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