[HTTP/OAS] Add description in create rule request body

[lcawl]

Summary

Relates to #180056

This PR moves some descriptions from the manually curated OpenAPI document (https://github.com/elastic/kibana/tree/main/x-pack/plugins/alerting/docs/openapi) into the code for automated OpenAPI docs generation.

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/alerting | jq > test.json

[apmmachine]

🤖 GitHub comments

Expand to view the GitHub comments

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)

[cnasikas]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 08d40bc

Metrics [docs]

✅ unchanged

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