[morningman]

Make the inserted NULL-handling rows agree with the displayed table and subsequent explanations, order the sample-table query by uid, and use the existing flag column in the empty-input example. Preserve the intended NULL-handling results.

Fixes #3904

Versions and languages: dev, 4.x and 3.x; English and Chinese.

Source verification:

• branch-3.1 be/src/vec/aggregate_functions/aggregate_function_retention.h ORs each input event and combines it with the first event
• The table defines uid, flag and flag2, so date is not a valid expression in the empty-input query
• Rows (0,true,NULL),(1,false,false) match the existing output and explanations for examples 5–7

Validation:

• Compared corrected input rows with all downstream expected results
• Confirmed empty-input query references a defined column and filters every inserted row
• Checked identical changes across six active EN/ZH pages
• git diff --check

Review:

• Goal: correct the reported documentation issue; the changed content directly addresses it.
• Scope: targeted documentation edits.
• Information architecture: active version and locale counterparts inspected; archived snapshots unchanged.
• Navigation: no paths, slugs, anchors or sidebar entries changed.
• Frontend/build: no framework, styling or build-script changes.
• Validation limits: no live Doris execution or full website build performed.
• Other concerns: No additional correctness, usability, accessibility, SEO or maintainability issues found in the changed content.