[FEATURE] Document new experimental ingestion streaming APIs

[reta]

Is your feature request related to a problem?

The new experimental ingestion streaming APIs are feature complete and need API specs.

What solution would you like?

Document new experimental ingestion streaming APIs.

The APIs use NDJSON (Newline delimited JSON, https://github.com/ndjson/ndjson-spec) which fits perfectly to the streaming use cases: each chunk sent along the request represents distinct bulk operation/action (index, create, delete, and update).

What alternatives have you considered?

N/A

Do you have any additional context?

• [Streaming Indexing] Introduce bulk HTTP API streaming flavour OpenSearch#9070

[reta]

@dblock could you please assign it to me? it is scheduled to 2.17.0, thank you!