add project_cost field documentation

Jetbuilt/jetbuilt#15535

Author: shawn-jb

source/includes/customers/projects/_items.md

@@ -20,6 +20,7 @@
       "mapp": "699.0",
       "price": "699.0",
       "cost": "433.38",
+      "project_cost": "433.38",
       "currency_iso": "USD",
       "short_description": "Subwoofer Gloss BlackWireless subwoofer",
       "shipping_price": "0.0",
@@ -224,6 +225,9 @@ Parameter | Description
 PROJECT_ID | The ID of the project
 page | A specific page of line item results. All factors are always returned.
 
+### "Cost" vs "Project Cost"
+The "Project Cost" is the item's cost in the project currency.
+
 ## Create a Project Item
 <%
   data =