Update batching+concurrency docs

[lkasinathan]

Right now, it says the following, all of which is categorically untrue.

However, the concurrency key option is ignored when batching is enabled. This means:

• If you configure concurrency: { limit: 1, key: "event.data.user_id" } with batching, the key expression will have no effect
• Batches will be processed one at a time globally (based on the limit), not per unique key value
• The concurrency limit applies to all batches regardless of what key values the events contain

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