api-ref: document sorting for log management

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 log endpoints.

Change-Id: I3f8a92348c54171e1dd6f7b6471e67df40d3d789
Partial-Bug: #1749024

api-ref/source/v2/logging.inc

@@ -42,6 +42,8 @@ Request
    - resource_id: resource_log_id-query
    - target_id: resource_target_log_id-query
    - enabled: log_enabled-query
+   - sort_dir: sort_dir
+   - sort_key: log-sort_key
    - fields: fields
 
 Response Parameters

api-ref/source/v2/parameters.yaml

@@ -485,6 +485,22 @@ ip_version-query:
   in: query
   required: false
   type: integer
+log-sort_key:
+  description: |
+    Sorts by a log attribute. You can specify multiple pairs of sort key
+    and sort direction query parameters. The sort keys are limited to:
+
+    - ``enabled``
+    - ``event``
+    - ``id``
+    - ``name``
+    - ``project_id``
+    - ``resource_id``
+    - ``resource_type``
+    - ``target_id``
+  in: query
+  required: false
+  type: string
 log_enabled-query:
   description: |
     Filter the log list result based on this log object is enabled (``true``)