chore(docs): Updated API Reference (v2) (#9148)

Automated changes by [create-pull-request](https://github.com/peter-evans/create-pull-request) GitHub action

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

www/apps/api-reference/specs/admin/components/schemas/AdminCampaign.yaml

@@ -30,15 +30,15 @@ properties:
   campaign_identifier:
     type: string
     title: campaign_identifier
-    description: The campaign's campaign identifier.
+    description: The campaign's identifier.
   starts_at:
     type: string
     title: starts_at
-    description: The campaign's starts at.
+    description: The date and time that the campaign starts.
   ends_at:
     type: string
     title: ends_at
-    description: The campaign's ends at.
+    description: The date and time that the campaign ends.
   budget:
     type: object
     description: The campaign's budget.
@@ -55,7 +55,11 @@ properties:
         description: The budget's ID.
       type:
         type: string
-        description: The budget's type.
+        description: >
+          The budget's type. `spend` means the limit is set on the total amount
+          discounted by the campaign's promotions; `usage` means the limit is
+          set on the total number of times the campaign's promotions can be
+          used.
         enum:
           - spend
           - usage
@@ -70,4 +74,8 @@ properties:
       used:
         type: number
         title: used
-        description: The budget's used.
+        description: >
+          How much of the budget has been used. If the limit is `spend`, this
+          property holds the total amount discounted so far. If the limit is
+          `usage`, it holds the number of times the campaign's promotions have
+          been used so far.