[NightG1owZ]

What changed

Documents the tag-based environment variable overrides for region_engine on datanode and standalone, added in GreptimeTeam/greptimedb#9700 (merged; the feature is on main, i.e. 1.4.0, and is not in release/v1.3).

• #### Environment Variable Rules: new paragraph on options whose value is a list of named entries. They are addressed by the entry name, not by its position in the list (GREPTIMEDB_DATANODE__REGION_ENGINE__MITO__GLOBAL_WRITE_BUFFER_REJECT_SIZE=4GB); a numeric index is rejected; region_engine defaults to mito and file while the metric engine is always registered; and once a [[region_engine]] entry appears in the configuration file the whole list comes from the file, so these variables are ignored for it.
• ### Region engine options: corrected the stale sentence that said only mito and metric options are available, and added a cross-reference to the environment variable rules.

The wording follows the merged config/config.md and the resolution of the Copilot review note on #9700 ("describe mito and file as the default configuration entries, not the only enabled engines"): the default configuration lists mito and file, and metric is always registered, so a metric entry only supplies its options rather than enabling the engine.

Scope

• Documentation versions: Nightly only (docs/). The option does not exist in any released GreptimeDB version: src/common/config/src/tagged_seq.rs is absent from release/v1.3, and the workspace on main is 1.4.0-alpha.0. No versioned_docs/ changes and no backport.
• Languages: English and Chinese, updated together.

Verification

DOC_LANG=en pnpm check:links   # exit 0, no broken links or anchors
DOC_LANG=zh pnpm check:links   # exit 0, no broken links or anchors
git diff --check               # clean

Rendered route (Nightly): /nightly/user-guide/deployments-administration/configuration/. Checked in the built HTML that both new paragraphs are present, that the old "Currently, options for mito and metric region engines are available" sentence is gone, and that the #environment-variable-rules anchor target exists.

Checklist

• I verified the content against the applicable GreptimeDB version. (Against main / 1.4.0: tagged_seq.rs, the region_engine defaults in src/datanode/src/config.rs and src/standalone/src/options.rs, the metric engine construction in src/datanode/src/datanode.rs, and the tests in src/cmd/tests/load_config_test.rs.)
• I updated the relevant documentation versions and languages, or explained why not. (Nightly EN+ZH; no versioned docs because no release contains the feature.)
• I checked changed links and anchors.
• I updated navigation when the document structure changed. (No structure change: no page added, moved or removed, and no heading changed.)

[fengjiachun]

LGTM