docs: document internal events schema reconciliation

[WenyXu]

What changed

Documents the GreptimeDB #8930 behavior tracked by #2774: event writes can create and additively reconcile the internal greptime.greptime_private.events table when the server-side global auto_create_table setting is false. Clarifies that the exception does not apply to user tables.

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

Verification

• pnpm build
• DOC_LANG=zh pnpm build
• git diff --check
• Reviewed the behavior against GreptimeDB #8930 and the aggregate documentation diff.

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.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.