Add documentation about when the application API was added

doc/api/applications.md

 # Applications API
 
+> [Introduced][ce-8160] in GitLab 10.5
+
+[ce-8160]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8160
+
 ## Create a application
 
 Create a application by posting a JSON payload.

lib/api/applications.rb

@@ -5,6 +5,7 @@ module API
 
     resource :applications do
       desc 'Create a new application' do
+        detail 'This feature was introduced in GitLab 10.5'
         success Entities::Application
       end
       params do