api-ref: Fix parameters about trusted certificate IDs

Fix the location of the 'trusted_image_certificates'
parameter in the parameter list
of the request body in the "Create Server" API.

Add an additional description that explains
the value is null if the trusted certificate IDs are not set
in the description for the 'trusted_image_certificates' parameter
in the response body.

Change-Id: Ibd3763044068c64b88a28d6c541c569233e112f8
Closes-Bug: #1777394

api-ref/source/parameters.yaml

@@ -5751,7 +5751,8 @@ server_trusted_image_certificates_resp:
   description: |
     A list of trusted certificate IDs, that were used during image signature
     verification to verify the signing certificate. The list is restricted
-    to a maximum of 50 IDs.
+    to a maximum of 50 IDs. The value is ``null`` if trusted certificate IDs
+    are not set.
   in: body
   required: true
   type: array

api-ref/source/servers.inc

@@ -372,6 +372,7 @@ Request
   - user_data: user_data
   - description: server_description
   - tags: server_tags_create
+  - trusted_image_certificates: server_trusted_image_certificates_create_req
   - os:scheduler_hints: os:scheduler_hints
   - os:scheduler_hints.build_near_host_ip: os:scheduler_hints_build_near_host_ip
   - os:scheduler_hints.cidr: os:scheduler_hints_cidr
@@ -381,7 +382,6 @@ Request
   - os:scheduler_hints.query: os:scheduler_hints_query
   - os:scheduler_hints.same_host: os:scheduler_hints_same_host
   - os:scheduler_hints.target_cell: os:scheduler_hints_target_cell
-  - trusted_image_certificates: server_trusted_image_certificates_create_req
 
 **Example Create Server**