Put opensearch-client configuration options into a separate section.

[MaxKsyunz]

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Describe the issue you're reporting

The following options relate to opensearch go client specifically. Normally they should be left at default values.
Make this clear in the documentation to avoid confusion.

- `num_workers` (optional): Number of workers publishing bulk requests concurrently.
- `flush`: Event bulk buffer flush settings
  - `bytes` (default=5242880): Write buffer flush limit.
  - `interval` (default=30s): Write buffer time limit.
- `retry`: Event retry settings
  - `enabled` (default=true): Enable/Disable event retry on error. Retry
    support is enabled by default.
  - `max_requests` (default=3): Number of HTTP request retries.
  - `initial_interval` (default=100ms): Initial waiting time if a HTTP request failed.
  - `max_interval` (default=1m): Max waiting time if a HTTP request failed.