Improve documentation for asset_url

Updated CHANGELOG.md with entry about :host in asset_url

actionview/CHANGELOG.md

@@ -87,5 +87,9 @@
 
     *Piotr Chmolowski, Łukasz Strzałkowski*
 
+*   Allow custom `:host` option to be passed to `asset_url` helper that
+    overwrites `config.action_controller.asset_host` for particular asset.
+
+    *Hubert Łępicki*
 
 Please check [4-1-stable](https://github.com/rails/rails/blob/4-1-stable/actionview/CHANGELOG.md) for previous changes.

actionview/lib/action_view/helpers/asset_url_helper.rb

@@ -147,7 +147,14 @@ module ActionView
 
       # Computes the full URL to an asset in the public directory. This
       # will use +asset_path+ internally, so most of their behaviors
-      # will be the same.
+      # will be the same. If :host options is set, it overwrites global
+      # +config.action_controller.asset_host+ setting.
+      #
+      # All other options provided are forwarded to +asset_path+ call.
+      #
+      #   asset_url "application.js"                                 # => http://example.com/application.js
+      #   asset_url "application.js", host: "http://cdn.example.com" # => http://cdn.example.com/javascripts/application.js
+      #
       def asset_url(source, options = {})
         path_to_asset(source, options.merge(:protocol => :request))
       end

guides/source/asset_pipeline.md

@@ -913,6 +913,14 @@ that it plays nicely with the pipeline. You may find quirks related to your
 specific set up, you may not. The defaults nginx uses, for example, should give
 you no problems when used as an HTTP cache.
 
+If you want to serve only some assets from your CDN, you can use custom
+`:host` option of  `asset_url` helper, which overwrites value set in
+`config.action_controller.asset_host`.
+
+```ruby
+asset_url 'image.png', :host => 'http://cdn.example.com'
+```
+
 Customizing the Pipeline
 ------------------------