[8.15] [DOCS] Clarify access to alerting system indices (#191114)

[kibanamachine]

Backport

This will backport the following commits from main to 8.15:

• [DOCS] Clarify access to alerting system indices (#191114)

Questions ?

Please refer to the Backport tool documentation

[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.}