[HTTP/OAS] Add ML sync API description

[lcawl]

Summary

Relates to #180056

This PR adds a string that gets output as the operation summary in the generated OpenAPI document.

How to test

Per #181277:

1. Start Elasticsearch (e.g. yarn es snapshot --license trial)
2. Add server.oas.enabled: true to kibana.dev.yml
3. Start Kibana (e.g. yarn start --no-base-path)
4. curl -s -uelastic:changeme http://localhost:5601/api/oas\?pathStartsWith\=/api/ml | jq > test.json

The output will contain info like this:

   "paths": {
    "/api/ml/saved_objects/sync": {
      "get": {
        "summary": "Synchronize machine learning saved objects",

[apmmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

LGTM

[kibana-ci]

💚 Build Succeeded

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• Commit: d48b3cd

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream