Correct link to Merge trains docs on MR widget

Closes https://gitlab.com/gitlab-org/gitlab/issues/34406

See merge request https://gitlab.com/gitlab-org/gitlab/merge_requests/19726

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -101,3 +101,5 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
                               merged_by: merge_event&.author)
   end
 end
+
+MergeRequestPollCachedWidgetEntity.prepend_if_ee('EE::MergeRequestPollCachedWidgetEntity')

changelogs/unreleased/34406-link-to-merge-trains-documentation-on-mr-widget-is-incorrect.yml

+---
+title: Correct link to Merge trains documentation on MR widget
+merge_request: 19726
+author:
+type: changed

ee/app/serializers/ee/merge_request_poll_cached_widget_entity.rb

+# frozen_string_literal: true
+
+module EE
+  module MergeRequestPollCachedWidgetEntity
+    extend ActiveSupport::Concern
+
+    prepended do
+      expose :merge_train_when_pipeline_succeeds_docs_path do |merge_request|
+        presenter(merge_request).merge_train_when_pipeline_succeeds_docs_path
+      end
+    end
+  end
+end

ee/spec/controllers/ee/projects/merge_requests/content_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::MergeRequests::ContentController do
+  let(:project) { create(:project, :repository) }
+  let(:user) { create(:user) }
+  let(:merge_request) { create(:merge_request, target_project: project, source_project: project) }
+
+  before do
+    sign_in(user)
+  end
+
+  def do_request(action = :cached_widget)
+    get action, params: {
+      namespace_id: project.namespace.to_param,
+      project_id: project,
+      id: merge_request.iid,
+      format: :json
+    }
+  end
+
+  context 'user has access to the project' do
+    before do
+      expect(::Gitlab::GitalyClient).to receive(:allow_ref_name_caching).and_call_original
+
+      project.add_maintainer(user)
+    end
+
+    describe 'GET cached_widget' do
+      it 'renders widget MR entity as json' do
+        do_request
+
+        expect(response).to match_response_schema('entities/merge_request_poll_cached_widget')
+      end
+    end
+  end
+end

ee/spec/fixtures/api/schemas/entities/merge_request_poll_cached_widget.json

+{
+  "type": "object",
+  "allOf": [
+    { "$ref": "../../../../../../spec/fixtures/api/schemas/entities/merge_request_poll_cached_widget.json" },
+    {
+      "properties" : {
+        "merge_train_when_pipeline_succeeds_docs_path": { "type": "string" }
+      }
+    }
+  ]
+}