[DOC] Enabling explainability for hybrid query

[martin-gaievski]

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• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
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Tell us about your request.

Existing documentation page for hybrid query (https://opensearch.org/docs/latest/search-plugins/hybrid-search/) should be updated, the new addition should mention that hybrid query will support the explain. Describe the explain option for hybrid queries, noting that it enables detailed visibility into score composition. When enabled, explain provides a breakdown of how scores from individual subqueries are normalized, combined, and contribute to the overall ranking.

Setup Requirement
To view explain details in responses, users must configure a new response processor within their search pipeline. This configuration is required to process and output the detailed scoring information for hybrid queries.

Usage with Full Query
The explain feature operates only when requested for the entire hybrid query (not for specific document IDs). Include a note that explain is triggered using the explain request parameter at the query level. The response will detail:

Score normalization and combination processes.
Calculations for each subquery’s score and their influence on the final score.
Explain by Document ID
Clarify that requesting explain for a specific document ID will only show the score breakdown at the individual query level, without detailing score normalization or combination. This option provides origin-level scoring insights without the combined explanation.

As development is ongoing we can give only draft of the request/response that are involved into this feature

Version:
Planned to be released in 2.19

What other resources are available?
GH issue: opensearch-project/neural-search#658
RFC for explain in hybrid query feature: opensearch-project/neural-search#905