feat: explain FIFO allocation of fixed Discount Amount on Sales Order (backport #56436) (#57062)

* feat: explain FIFO allocation of fixed Discount Amount on Sales Order (#56436)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Diptanil Saha <diptanil@frappe.io>
(cherry picked from commit 62fed1d562)

# Conflicts:
#	erpnext/selling/doctype/sales_order/sales_order.json

* chore: resolved conflicts

---------

Co-authored-by: Mohammad Umair Sayed <umair_sayyed@yahoo.com>
Co-authored-by: Diptanil Saha <diptanil@frappe.io>

erpnext/selling/doctype/sales_order/sales_order.json

@@ -893,13 +893,15 @@
    "print_hide": 1
   },
   {
+   "description": "Applying a Discount Amount? When this Sales Order is partially fulfilled through multiple Delivery Notes and Sales Invoices, the Discount Amount is allocated on a FIFO basis. The earlier transactions receive a larger share of the discount. To spread the discount proportionally across item prices, use Additional Discount Percentage instead.",
    "fieldname": "discount_amount",
    "fieldtype": "Currency",
    "hide_days": 1,
    "hide_seconds": 1,
    "label": "Additional Discount Amount",
    "options": "currency",
-   "print_hide": 1
+   "print_hide": 1,
+   "show_description_on_click": 1
   },
   {
    "fieldname": "base_grand_total",
@@ -1766,7 +1768,7 @@
  "idx": 105,
  "is_submittable": 1,
  "links": [],
- "modified": "2026-06-21 12:46:13.250145",
+ "modified": "2026-06-24 12:00:00.000000",
  "modified_by": "Administrator",
  "module": "Selling",
  "name": "Sales Order",