[HTTP/OAS] Add description for task manager health API

[lcawl]

Summary

Relates to #180056

This PR an operation summary for the [get task manager healthAPI](https://www.elastic.co/guide/en/kibana/current/task-manager-api-health.html 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/task_manager | jq > test.json

The output will contain info like this:

 "paths": {
    "/api/task_manager/_health": {
      "get": {
        "summary": "Get task manager health",

[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/response-ops (Team:ResponseOps)

[ymao1]

LGTM

[kibana-ci]

💚 Build Succeeded

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• Commit: 6483291

Metrics [docs]

✅ unchanged

History

• 💛 Build #212446 was flaky b4a04ee

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@elasticmachine merge upstream