[HTTP/OAS] Add descriptions for upgrade assistant APIs

[lcawl]

Summary

Relates to #180056

This PR adds operation summaries for the upgrade assistant APIs 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 -u elastic http://localhost:5601/api/oas\?pathStartsWith\=/api/upgrade_assistant | jq > test.json

    "/api/upgrade_assistant/reindex/{indexName}": {
      "post": {
        "summary": "Start or resume reindex",

[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!)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 0aa3675

Failed CI Steps

• FTR Configs #45

Test Failures

• [job] [logs] FTR Configs #45 / endpoint "before all" hook in "endpoint"

Metrics [docs]

✅ unchanged

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