Document Advanced Search migration helpers for developers

We have a few helper files available to make writing Advanced Search migrations easier. We should document how to use these in the creating a new advanced search migration section of our help files. Specifically the helpers in:

• ee/app/workers/concerns/elastic/migration_backfill_helper.rb
• ee/app/workers/concerns/elastic/migration_helper.rb

The following discussion from !68946 (merged) should be addressed:

• @dgruzd started a discussion: (+4 comments)

  @terrichu I believe this is ready for an initial backend review. Are you available for one? Thank you!