Improve documentation for Linear Top Level Normalizer

[mridula-s109]

This PR adds a clarification in the linear top level normalizer.

Follow up from PR: #129693

[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/linear-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

[mridula-s109]

@leemthompo @dimitris-athanasiou can you please approve the finalised version or please let me know if any further changes are required?

[leemthompo]

LGTM with a final tweak. I recommend always checking the URL preview to make sure that docs changes make sense for the reader, especially when using these inline applies_to tags which need to clearly target a specific section :)