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api: address batch endpoint review (docs + client metadata)

- Document that TransactionBatchItem.Code is always 0 for submitted items (a
  non-zero CheckTx code sends the item to Failed instead) (P1).
- Make the metadata-pin loop's index correspondence explicit: it relies on the
  fail-fast invariant that submitted items are the leading prefix of the input,
  so Submitted[i] maps to Transactions[i] (P2).
- apiclient.SendTxBatch now takes []api.TransactionPayload, so callers can supply
  per-item Metadata for NewProcess txs (validated + pinned by the server), closing
  the gap where the helper only accepted raw payloads (P3).