[doc] running-difference is a deprecated zh-only orphan in 2.1/3.x (removed in dev/4.x); example references undefined table

[boluor]

While verifying SQL-function example pages I found running-difference is in an inconsistent state across versions.

Findings

1. The function is deprecated — the page itself says so ("此函数会在后续版本中废弃，推荐使用窗口函数"), and the page has been removed entirely from dev and version-4.x (both EN and ZH).
2. In version-2.1 and version-3.x the page survives only in Chinese (i18n/zh-CN/.../version-{2.1,3.x}/sql-manual/sql-functions/scalar-functions/numeric-functions/running-difference.md) — there is no EN counterpart in those versions (only the EOL 1.2/2.0 trees have one).
3. The description's example references an undefined table t:

   SELECT running_difference(x) FROM t ORDER BY k;
   SELECT x - lag(x, 1, 0) OVER (ORDER BY k) FROM t;

   These are an illustrative function-form-vs-window-form comparison (no output shown), so they aren't runnable as-is.
4. The worked example (running_difference_test, BUCKETS 3) is documented as non-deterministic ("结果是不稳定的", the row order used during computation may differ from the returned order), so its printed output cannot be reliably reproduced.

Suggestion

Since the function is deprecated and the page is already gone from dev/4.x, the cleanest fix is to remove the orphaned zh-only running-difference page from version-2.1 and version-3.x to match dev/4.x, rather than retrofit a table for a deprecated, non-deterministic example. (If keeping it is preferred, the FROM t snippet should be marked illustrative and the worked example reworked to be deterministic.)

Happy to open a PR removing the 2.1/3.x zh pages if that's the preferred direction.

[zclllyybb]

Breakwater-GitHub-Analysis-Slot: slot_ae4547a941ce

I checked the live issue and refreshed apache/doris-website upstream/master before reviewing the docs tree. The report looks valid.

Evidence from the current docs repository:

• running-difference.md exists in 2.1 and 3.x only under i18n/zh-CN/docusaurus-plugin-content-docs/version-{2.1,3.x}/.../numeric-functions/.
• There is no matching English versioned_docs/version-2.1 or versioned_docs/version-3.x page, and I did not find a current/dev or version-4.x running-difference page.
• The sidebar references I found are only for the older 1.2 and 2.0 pages, not for the 2.1/3.x zh-CN files, so these 2.1/3.x files appear to be orphaned localized content.
• The remaining 2.1/3.x zh-CN pages explicitly describe the function as unstable and deprecated, and the FROM t comparison snippet is illustrative but not runnable because t, x, and k are not defined.

My recommendation is to remove the two orphaned zh-CN pages from version-2.1 and version-3.x. That keeps 2.1/3.x consistent with current/dev and 4.x, and avoids investing new example work in a deprecated function whose documented behavior is order-dependent.

If maintainers prefer to keep historical documentation for this deprecated function in 2.1/3.x, then the safer fallback is not just adding a missing table. The page should also mark the FROM t snippet as illustrative/non-runnable and either remove the printed running_difference_test outputs or rework the examples around deterministic window-function replacements.

I did not find an open PR for running-difference / #3902 at the time of review. No runtime logs or profiles are needed for this issue; the only missing decision is the docs policy choice: remove these deprecated orphaned localized pages, or keep them and rewrite the examples accordingly.