Bulk insert docs: document `upsert` / `skip_duplicates`

What does this MR do?

In !27215 (merged) we introduced a new parameter, skip_duplicates, as well as support for bulk_upserts. This MR updates the docs accordingly.

I also added a line about jsonb since we found out it does not currently work with bulk inserts.

Related issues

MR: !27215 (merged)

I don't believe we had an issue for this.

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