Refactor settings API documentation [ci skip]

doc/api/settings.md

 # Application settings
 
-This API allows you to read and modify GitLab instance application settings. 
+These API calls allow you to read and modify GitLab instance application
+settings as appear in `/admin/application_settings`. You have to be an
+administrator in order to perform this action.
 
+## Get current application settings
 
-## Get current application settings: 
+List the current application settings of the GitLab instance.
 
 ```
 GET /application/settings
 ```
 
+```bash
+curl -H "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v3/application/settings
+```
+
+Example response:
+
 ```json
 {
-  "id": 1,
-  "default_projects_limit": 10,
-  "signup_enabled": true,
-  "signin_enabled": true,
-  "gravatar_enabled": true,
-  "sign_in_text": "",
-  "created_at": "2015-06-12T15:51:55.432Z",
-  "updated_at": "2015-06-30T13:22:42.210Z",
-  "home_page_url": "",
-  "default_branch_protection": 2,
-  "twitter_sharing_enabled": true,
-  "restricted_visibility_levels": [],
-  "max_attachment_size": 10,
-  "session_expire_delay": 10080,
-  "default_project_visibility": 0,
-  "default_snippet_visibility": 0,
-  "restricted_signup_domains": [],
-  "user_oauth_applications": true,
-  "after_sign_out_path": ""
+   "default_projects_limit" : 10,
+   "signup_enabled" : true,
+   "id" : 1,
+   "default_branch_protection" : 2,
+   "restricted_visibility_levels" : [],
+   "signin_enabled" : true,
+   "twitter_sharing_enabled" : true,
+   "after_sign_out_path" : null,
+   "max_attachment_size" : 10,
+   "user_oauth_applications" : true,
+   "updated_at" : "2016-01-04T15:44:55.176Z",
+   "session_expire_delay" : 10080,
+   "home_page_url" : null,
+   "default_snippet_visibility" : 0,
+   "restricted_signup_domains" : [],
+   "created_at" : "2016-01-04T15:44:55.176Z",
+   "default_project_visibility" : 0,
+   "gravatar_enabled" : true,
+   "sign_in_text" : null
 }
 ```
 
-## Change application settings: 
-
-
+## Change application settings
 
 ```
 PUT /application/settings
 ```
 
-Parameters:
-
-- `default_projects_limit` - project limit per user
-- `signup_enabled` - enable registration
-- `signin_enabled` - enable login via GitLab account
-- `gravatar_enabled` - enable gravatar
-- `sign_in_text` - text on login page
-- `home_page_url` - redirect to this URL when not logged in
-- `default_branch_protection` - determine if developers can push to master
-- `twitter_sharing_enabled` - allow users to share project creation in twitter
-- `restricted_visibility_levels` - restrict certain visibility levels
-- `max_attachment_size` - limit attachment size
-- `session_expire_delay` - session lifetime
-- `default_project_visibility` - what visibility level new project receives
-- `default_snippet_visibility` - what visibility level new snippet receives
-- `restricted_signup_domains` - force people to use only corporate emails for signup
-- `user_oauth_applications` - allow users to create oauth applications
-- `after_sign_out_path` - where redirect user after logout
+| Attribute | Type | Required | Description |
+| --------- | ---- | :------: | ----------- |
+| `default_projects_limit` | integer  | no | Project limit per user. Default is `10` |
+| `signup_enabled`    | boolean | no  | Enable registration. Default is `true`. |
+| `signin_enabled`    | boolean | no  | Enable login via a GitLab account. Default is `true`. |
+| `gravatar_enabled`  | boolean | no  | Enable Gravatar |
+| `sign_in_text`      | string  | no  | Text on login page |
+| `home_page_url`     | string  | no  | Redirect to this URL when not logged in |
+| `default_branch_protection` | integer | no | Determine if developers can push to master. Can take `0` _(not protected, both developers and masters can push new commits, force push or delete the branch)_, `1` _(partially protected, developers can push new commits, but cannot force push or delete the branch, masters can do anything)_ or `2` _(fully protected, developers cannot push new commits, force push or delete the branch, masters can do anything)_ as a parameter. Default is `1`. |
+| `twitter_sharing_enabled` | boolean | no | Allow users to share project creation on Twitter |
+| `restricted_visibility_levels` | array of integers | no | Selected levels cannot be used by non-admin users for projects or snippets. Can take `0` _(Private)_, `1` _(Internal)_ and `2` _(Public)_ as a parameter. Default is null which means there is no restriction. |
+| `max_attachment_size` | integer | no | Limit attachment size in MB |
+| `session_expire_delay` | integer | no | Session duration in minutes. GitLab restart is required to apply changes |
+| `default_project_visibility` | integer | no | What visibility level new projects receive. Can take `0` _(Private)_, `1` _(Internal)_ and `2` _(Public)_ as a parameter. Default is `0`.|
+| `default_snippet_visibility` | integer | no | What visibility level new snippets receive. Can take `0` _(Private)_, `1` _(Internal)_ and `2` _(Public)_ as a parameter. Default is `0`.|
+| `restricted_signup_domains` | array of strings | no | Force people to use only corporate emails for sign-up. Default is null, meaning there is no restriction. |
+| `user_oauth_applications` | boolean | no | Allow users to register any application to use GitLab as an OAuth provider |
+| `after_sign_out_path` | string | no | Where to redirect users after logout |
 
-All parameters are optional. You can send only one that you want to change.
+```bash
+curl -X PUT -H "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v3/application/settings?signup_enabled=false&default_project_visibility=1
+```
 
+Example response:
 
 ```json
 {
@@ -79,7 +89,7 @@ All parameters are optional. You can send only one that you want to change.
   "restricted_visibility_levels": [],
   "max_attachment_size": 10,
   "session_expire_delay": 10080,
-  "default_project_visibility": 0,
+  "default_project_visibility": 1,
   "default_snippet_visibility": 0,
   "restricted_signup_domains": [],
   "user_oauth_applications": true,