Update multi-field query format docs to indicate serverless support

[Mikep86]

No description provided.

[github-actions]

🔍 Preview links for changed docs

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

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[leemthompo]

LGTM!

[leemthompo]

In theory, it's implicit that everything is available on serverless, if the frontmatter already indicates that the page applies to serverless (and everything is GA by default on serverless, unless specified too)

But folks are working on making these more intuitive and this doesn't hurt in the meantime

[Mikep86]

Thanks for the clarification @leemthompo! Do you think we need this PR then?

[leemthompo]

technically no, I guess as long as the page level applicability is set

[kderusso]

[Mikep86]

Closing without merging because the relevant pages wholly apply to serverless, and thus each subsection implicitly applies to serverless as well.