docs(s3 backend): add caution around skip_* flags; clarify testing/compat intent

[youming1970]

Docs-only hardening; no behavior change.

• Add a caution block explaining that skip_* flags (skip_credentials_validation, skip_requesting_account_id, skip_metadata_api_check) are primarily intended for testing or S3-compatible implementations and may bypass AWS validations and metadata checks.
• Mark these options as "Not recommended for production".
• Encourage use of standard AWS mechanisms (IAM, STS) and validated TLS, and advise providing credentials via environment variables or secret managers.

Related to hashicorp/terraform#37568.

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