docs: port discard_unflushed docs to v1.2

[evenyag]

What changed

• Port the discard_unflushed ADMIN documentation to the English and Chinese v1.2 reference pages.
• Document the table and Region ID forms, data-loss warning, supported targets, and usage constraints.

Scope

• Documentation versions: 1.2
• Languages: English, Chinese

Verification

• git diff --check
• Builds and link checks were not run, per request.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed. (Not applicable; no navigation changes.)

[cloudflare-workers-and-pages]

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