[DOC] Document the use of protobuf feature flag

[VachaShah]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
We are adding an experimental feature in 2.13 to enable protobuf for normal searches (QUERY_THEN_FETCH without aggregations). To enable this feature, the feature flag needs to be turned on:

opensearch.experimental.feature.search_with_protobuf.enabled=true

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

opensearch-project/OpenSearch#11910

[getsaurabh02]

Meta : opensearch-project/OpenSearch#10684

[hdhalter]

Thanks, @VachaShah , @getsaurabh02 ! Who will be drafting the doc PR?

[VachaShah]

@hdhalter I can draft the doc PR. @getsaurabh02 Do you know what would be the best place to add this feature flag info? I know there is a page on the website for how to use feature flags, but is there a place where the feature flags are listed?

[VachaShah]

This is not part of 2.13.0 anymore.

[hdhalter]

Hi @VachaShah , is this on track for 2.14?