Update merge request versions API to match styleguide

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/api/merge_requests.md

@@ -903,11 +903,16 @@ Get a list of merge request diff versions.
 GET /projects/:id/merge_requests/:merge_request_id/versions
 ```
 
-Parameters:
+| Attribute | Type    | Required | Description           |
+| --------- | ------- | -------- | --------------------- |
+| `id`      | integer | yes      | The ID of the project |
+| `merge_request_id` | integer | yes | The ID of the merge request |
 
-- `id` (required) - The ID of a project
-- `merge_request_id` (required) - The ID of MR
+```bash
+curl -H "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v3/projects/1/merge_requests/1/versions
+```
 
+Example response:
 
 ```json
 [{

lib/api/merge_request_diffs.rb

@@ -4,15 +4,16 @@ module API
     before { authenticate! }
 
     resource :projects do
-      # List merge requests diff versions
-      #
-      # Parameters:
-      #   id (required)               - The ID of a project
-      #   merge_request_id (required) - The ID of MR
-      #
-      # Example:
-      #   GET /projects/:id/merge_requests/:merge_request_id/versions
-      #
+      desc 'Get a list of merge request diff versions' do
+        detail 'This feature was introduced in GitLab 8.12.'
+        success Entities::MergeRequestDiff
+      end
+
+      params do
+        requires :id, type: Integer, desc: 'The ID of a project'
+        requests :merge_request_id, type: Integer, desc: 'The ID of a merge request'
+      end
+
       get ":id/merge_requests/:merge_request_id/versions" do
         merge_request = user_project.merge_requests.
           find(params[:merge_request_id])