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.
@@ -25,7 +29,7 @@ curl --request POST --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" --data "name=
 Example response:
 
 ```json
-{  
+{
     "application_id": "5832fc6e14300a0d962240a8144466eef4ee93ef0d218477e55f11cf12fc3737",
     "secret": "ee1dd64b6adc89cf7e2c23099301ccc2c61b441064e9324d963c46902a85ec34",
     "callback_url": "http://redirect.uri"

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