[doc](workload) Document BE-side username condition for workload policy

[wenzhenghu]

Summary:

• Document that username can be used with BE-side runtime metrics to trigger cancel_query starting from Doris 4.1.3.
• Add dev and 4.x examples in both English and Chinese.
• Clarify username only supports equality and cannot be empty.

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