RS: Documented CRDB default_db_config usage and config requirements

[rrelledge]

Note

Low Risk
Documentation-only changes with no runtime or API behavior changes; incorrect guidance could mislead operators but does not alter product code.

Overview
Documents how Active-Active (CRDB) settings should be applied via default_db_config versus per-instance db_config, and wires that guidance through the REST API, CLI, and memory/OOM topics.

The CRDB database_config reference is restructured into four buckets: settings that must use default_db_config (replication-breaking if mismatched), should use it (operational consistency), recommended defaults (including replication_oom_threshold_percent with a link to replication OOM protection), and fields that belong in per-instance db_config (certs, port, mTLS flags, etc.). Related pages (crdb, instance_info, PATCH/POST crdbs, and crdbs/updates) now point readers at that reference and clarify that updating default_db_config applies to existing and new instances while instance db_config overrides defaults.

Replication OOM tuning in Active-Active planning and memory limits no longer documents single-BDB PUT /v1/bdbs; it shows PATCH /v1/crdbs/{crdb_guid} with default_db_config.replication_oom_threshold_percent so the change applies across participating clusters. crdb-cli create/update descriptions for --default-db-config / file options now link to the same config reference.

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[github-actions]

DOC-6768

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6768/operate/rs/databases/active-active/planning/
https://redis.io/docs/staging/DOC-6768/operate/rs/databases/memory-performance/memory-limit/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/cli-utilities/crdb-cli/crdb/create/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/cli-utilities/crdb-cli/crdb/update/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/rest-api/objects/crdb/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/rest-api/objects/crdb/database_config/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/rest-api/objects/crdb/instance_info/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/rest-api/requests/crdbs/
https://redis.io/docs/staging/DOC-6768/operate/rs/references/rest-api/requests/crdbs/updates/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[andy-stark-redis]

Text LGTM.