Docs: Add weight parameter documentation for Weighted RRF retriever

[mridula-s109]

Summary

Adds missing documentation for the weight parameter introduced with the Weighted RRF retriever (GA'd in #130658).

Closes #136477

Changes

rrf-retriever.md (REST API Reference):

• Added sub-parameters section documenting retriever and weight fields
• Added weighted hybrid search example showing weight usage
• Follows linear-retriever.md documentation patterns

retrievers-examples.md (Examples Guide):

• Updated main RRF example to demonstrate weighted retrievers
• Shows semantic search (weight 2.0) and lexical search (weight 1.0)

Testing

• ./gradlew :docs:check passed (917 tasks completed)
• Documentation-only changes

Checklist

• Signed CLA
• Followed contributor guidelines
• Built locally with gradle check
• PR against main branch

Related

• Feature PR: Implement support for weighted rrf #130658
• Issue: Add missing weight parameter documentation for Weighted RRF retriever and update elasticsearch-specification #136477
• Next: elasticsearch-specification TypeScript update (separate PR)

[Copilot]

Pull Request Overview

This PR adds documentation for the weight parameter in the Weighted RRF retriever, which was previously introduced but lacked proper documentation. The changes provide comprehensive examples and parameter descriptions to help users understand how to adjust retriever influence in RRF ranking.

• Added detailed documentation for retriever and weight sub-parameters in the RRF retriever
• Added weighted hybrid search example showing different weight configurations
• Updated the main RRF example to demonstrate weighted retrievers with semantic vs lexical search weighting

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/reference/elasticsearch/rest-apis/retrievers/rrf-retriever.md	Added weight parameter documentation, sub-parameters section, and weighted hybrid search example
docs/reference/elasticsearch/rest-apis/retrievers/retrievers-examples.md	Updated main RRF example to use weighted format with semantic search having higher influence

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[elasticsearchmachine]

Pinging @elastic/search-relevance (Team:Search - Relevance)

[elasticsearchmachine]

Pinging @elastic/core-docs (Team:Docs)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/rest-apis/retrievers/retrievers-examples.md
• docs/reference/elasticsearch/rest-apis/retrievers/rrf-retriever.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
❌ Don't add new information that applies to a specific version without an applies_to tag
❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[leemthompo]

Hey @mridula-s109 remember per the comment, that we can't just overwrite info from a previous version, we need to use applies_to tags to make it clear what changed in 9.2 without deleting any information that's relevant to 9.0 + 9.1 :)

[mridula-s109]

Hey @mridula-s109 remember per the comment, that we can't just overwrite info from a previous version, we need to use applies_to tags to make it clear what changed in 9.2 without deleting any information that's relevant to 9.0 + 9.1 :)

Ah yes, it slipped my mind. i will update the changes accordingly.

[kderusso]

Looks good overall, some comments on vocabulary

[kderusso]

Lexical is a very search-practitioner specific term, that not everyone reading this documentation may know.

Suggested change

	In this example, we're giving the lexical search twice the influence of the semantic search (kNN):
	In this example, we're giving the lexical search (in the `standard` retriever) twice the influence of the semantic search (in the `knn` retriever):

[kderusso]

Can we make this clarification a little less ambiguous? I am not sure what "direct format" should mean and the example isn't helpful. Is there somewhere we could link for format documentation, or add additional explanation that is more clear?

[kderusso]

Maybe add something that if a weight is not specified all retrievers are equally weighted against each other?

[kderusso]

Same comment on usage of the term lexical.