Add details about agent namespaces cascading to integration policies

[kilfoyle]

With elastic/kibana#174776, agent policy namespaces will now cascade to integration policies unless otherwise specified. This adds a paragraph to clarify that behaviour in the docs:

[github-actions]

A documentation preview will be available soon.

Help us out by validating the Buildkite preview and reporting issues here.
Please also be sure to double check all images to ensure they are correct in the preview.

• 🔨 Buildkite builds
• 📚 HTML diff: Buildkite - Jenkins
• 📙 Preview: Buildkite - Jenkins
• 🧪 Buildkite vs Jenkins diff

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[criamico]

Looks good! Will it be visible only in 8.13 docs right?

[kilfoyle]

Looks good! Will it be visible only in 8.13 docs right?

Thanks @criamico! Yes, I'll merge this into main with no backports, so it'll appear only in the 8.13 and later docs.

[karenzone]

LGTM