[HTTP/OAS] Add descriptions for role management APIs

[lcawl]

Summary

Relates to #180056

This PR adds operation summaries for the role management APIs and user session management API 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/security/role | jq > test1.json
5. curl -s -u elastic http://localhost:5601/api/oas\?pathStartsWith\=/api/security/session | jq > test2.json
   The output will contain info like this:

  "paths": {
    "/api/security/role/{name}": {
      "get": {
        "summary": "Get a role",

[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!)

[azasypkin]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 2f8f05b

Metrics [docs]

✅ unchanged

History

• 💚 Build #212915 succeeded deca968
• 💔 Build #212891 failed e5bb58d
• 💔 Build #212601 failed 745f9e5

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

[azasypkin]

LGTM, thanks!