docs: clarify OpenSearch authentication setup

[PSR94]

Related Issues

• fixes Update authorization in OpenSeachDocumentStore example  #9171

Proposed Changes:

Clarifies that the http_auth value for OpenSearchDocumentStore depends on the deployment type:

• local Docker uses the OpenSearch admin user and the OPENSEARCH_INITIAL_ADMIN_PASSWORD value from the container setup
• Amazon OpenSearch Service should use the domain's configured authentication method instead of the local Docker password or IAM console password
• IAM-based Amazon OpenSearch Service access can use an AWS SigV4 auth object, such as AWSV4SignerAuth, with a link to the OpenSearch Python client documentation

The clarification is added to both the unstable docs page and the latest stable version-2.31 copy.

How did you test it?

• uvx hatch run pre-commit run --files docs-website/docs/document-stores/opensearch-document-store.mdx docs-website/versioned_docs/version-2.31/document-stores/opensearch-document-store.mdx
• npm run build from docs-website/
• git diff --check

Notes for the reviewer

Documentation-only change; no release note is included.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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[bogdankostic]

Hi @PSR94, thanks for this PR! Please sign the CLA for me to be able to review and merge it.