Improve ES::Model README suggestion for async indexing with after_save_commit

[trappist]

I got a lot of test failures using the original suggestion. First because newly created records weren't yet committed when the (test) worker ran, so I switched to after_commit. But after_commit also fires on destroy, so it would fail trying to reindex destroyed records. Rails 6 introduces after_save_commit for this purpose. Prior to Rails 6 it would be after_commit ..., on: [:create, :update], but if you needed to change the next line to after_commit ..., on: :destroy the latter callback would clobber the former, and this prevents that.

[cla-checker-service[bot]]

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