[8.17] Add restrictions for synthetic _source (backport #1622)

[mergify]

This adds the restrictions described in #1618 to the Enabling and disabling advanced indexing features for Fleet-managed data streams page.

I also removed some extra spacing which I must have added earlier by accident.

@lucabelluccini and @martijnvg, please check that I have the enable and disable code examples right, since I'm kind of guessing.

Closes: #1618

Previews:

---

---

---

This is an automatic backport of pull request #1622 done by [Mergify](https://mergify.com).

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

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.}