api-ref: document sorting parameters for address scope

API users can use the 'sort_key' and 'sort_dir' to do the sorting
on list result. This patch add documentation of these two parameters
for address scope resource.

Change-Id: I2016812fccc870900fb70fa9cf0358b98a7356b3
Closes-Bug: #1749024

api-ref/source/v2/address-scopes.inc

@@ -148,6 +148,7 @@ Request
    - project_id: project_id-query
    - ip_version: ip_version-query
    - shared: shared-query
+   - sort_key: address_scope-sort_key
    - fields: fields
 
 Response Parameters

api-ref/source/v2/parameters.yaml

@@ -304,6 +304,20 @@ vpnservice_id-path:
   required: true
   type: string
 # variables in query
+address_scope-sort_key:
+  description: |
+    Sorts by an address scope attribute. You can specify multiple pairs of sort
+    key and sort direction query parameters. The sort keys are limited to:
+
+    - ``id``
+    - ``ip_version``
+    - ``name``
+    - ``project_id``
+    - ``shared``
+    - ``tenant_id``
+  in: query
+  required: false
+  type: string
 address_scope_id-query:
   description: |
     Filter the subnet pool list result by the address scope that is assigned