[fix](doc) remove Routine Load flexible partial update from 3.x docs

[Mesut-Doner]

The version-3.x Routine Load manual documents unique_key_update_mode (including UPDATE_FLEXIBLE_COLUMNS) as a job property and ships a full "Flexible Partial Column Update" CREATE ROUTINE LOAD example. This feature does not exist in any 3.x release.

On Doris 3.1.4, creating such a job fails at property validation:

ERROR 1105 (HY000): errCode = 2, detailMessage = unique_key_update_mode is invalid property

unique_key_update_mode was added to Routine Load by apache/doris #59896 and #60128, merged to master in January 2026 and never backported to branch-3.1. On branch-3.1 the property is absent from the Routine Load whitelist (CreateRoutineLoadStmt.PROPERTIES_SET), so it is rejected at CREATE before any BE involvement. The property exists only on the Stream Load path in 3.x (StreamLoadTask), which is why the Stream Load and partial-column-update docs are unaffected.

Remove, from the English, Chinese, and Japanese version-3.x copies:

• the unique_key_update_mode row in the job_properties table
• the "Flexible Partial Column Update" example section

partial_columns (fixed-column partial update) is supported in 3.x Routine Load and is left in place. The dev and 4.x docs are unchanged because the feature exists there.

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync