Document required user field for user-scoped budget creation (#61625)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: canuckjacq <11161530+canuckjacq@users.noreply.github.com>
Co-authored-by: Sophie <29382425+sophietheking@users.noreply.github.com>

Author: 4 people

content/rest/billing/budgets.md

@@ -12,4 +12,25 @@ category:
   - Administer enterprises and billing
 ---
 
+> [!IMPORTANT]
+> The request body schema below is missing a required field. When `budget_scope` is `user`, you must also include a `user` field set to the {% data variables.product.company_short %} username the budget applies to. If you omit this field, the API returns `HTTP 400: Missing required fields: budget_entity_name`. For user-scoped budgets, `budget_entity_name` can be an empty string.
+
+The following example creates a user-scoped budget that limits a single user's monthly {% data variables.product.prodname_copilot_short %} {% data variables.product.prodname_ai_credits_short %} to $30 USD:
+
+```json
+{
+  "budget_amount": 30,
+  "prevent_further_usage": true,
+  "budget_scope": "user",
+  "budget_entity_name": "",
+  "budget_type": "BundlePricing",
+  "budget_product_sku": "ai_credits",
+  "budget_alerting": {
+    "will_alert": false,
+    "alert_recipients": []
+  },
+  "user": "USERNAME"
+}
+```
+
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