[IMP] purchase_invoice_new_picking_line: Improve description

Signed-off-by: Carmen Bianca BAKKER <[email protected]>

Author: carmenbianca

purchase_invoice_new_picking_line/readme/DESCRIPTION.rst

@@ -1,3 +1,3 @@
-When creating an invoice from incoming picking,
-this module also adds invoice lines for product
-that were not in the purchase order.
+When creating an invoice from a purchase order, this module also adds invoice
+lines for products that were in the order's pickings but not in the order
+itself.