api-ref: body verification for list server migrations

This completes the body verification for listing server
migrations.

Note that the code currently filters such that only in-progress
live migration records are returned. This was also only available
with microversion >= 2.23.

Part of blueprint api-ref-in-rst-ocata

Change-Id: I3a785dad690eee80ec1fd50ebc9fd3f7484dc344

api-ref/source/server-migrations.inc

@@ -12,6 +12,18 @@ List Migrations
 
 .. rest_method:: GET /servers/{server_id}/migrations
 
+Lists in-progress live migrations for a given server.
+
+.. note:: Microversion 2.23 or greater is required for this API.
+
+Policy defaults enable only users with the administrative role to perform
+this operation. Cloud providers can change these permissions through the
+``policy.json`` file.
+
+Normal response codes: 200
+
+Error response codes: unauthorized(401), forbidden(403), itemNotFound(404)
+
 Request
 -------