Add multi-tenancy disable instructions for OpenSearch

[pawelw1]

Added instructions to disable multi-tenancy for OpenSearch and OpenSearch Dashboards.

Description

Current documentation only instructs how to disable multi-tenancy in Opensearch Dashboards. Multi-tenancy in OpenSearch is enabled by default and must also be disabled when the workspaces feature is enabled.

Version

3.x

Checklist

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
  For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Thank you for submitting your PR. The PR states are In progress (or Draft) -> Tech review -> Doc review -> Merged.

Before you submit your PR for doc review, make sure the content is technically accurate. If you need help finding a tech reviewer, tag a maintainer.

When you're ready for doc review, tag the assignee of this PR. The doc reviewer may push edits to the PR directly or leave comments and editorial suggestions for you to address (let us know in a comment if you have a preference).

[kolchfa-aws]

@cwperks Could you please review this PR?

[cwperks]

I'm reluctant to add to documentation here. Personally, I'm not aligned with the current implementation of workspaces. We can add to the documentation for sure, but I will not be marking approval. You can see more information on why I have taken this stance here: opensearch-project/security#6119

[kolchfa-aws]

@SuZhou-Joe Could you weigh in on this PR?

[kkhatua]

@cwperks I'm inclined to agree with you, but it the ability to disable already exists. This is only documentation, so may be @pawelw1 should add a clear banner indicating warning about the security gap be an acceptable approach ??
@SuZhou-Joe thoughts since you approved this?

[kolchfa-aws]

Closing this PR for now until we get alignment on this.