Improve reprocessing task documentation

Change-Id: I51e50d1b4f7d5fbe7dcbda7ddcc8b8453eb5c439

api-ref/source/v2/task/reprocessing.inc

@@ -113,6 +113,7 @@ one can retrieve all reprocessing tasks scheduled for a scope.
 .. rest_parameters:: task/reprocessing_parameters.yml
 
    - scope_ids: scope_ids_query
+   - order: scope_ids_order_query
 
 Status codes
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api-ref/source/v2/task/reprocessing_parameters.yml

@@ -19,6 +19,15 @@ offset:
   type: int
   required: false
 
+scope_ids_order_query: &scope_ids_order_query
+  in: query
+  description: |
+    The order in which reprocessing tasks are returned. By default, we
+    order reprocessing tasks ``desc`` by their insertion order in the
+    database. The possible values are ``desc`` or ``asc``.
+  required: false
+  type: string
+
 scope_ids_query: &scope_ids_query
   in: query
   description: |