Add more CHANGELOG entries from #7927

Also remove references for RAILS_ASSET_ID from the guides

[ci skip]

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Remove support for the RAILS_ASSET_ID environment configuration
+    (no longer needed now that we have the asset pipeline).
+
+    *Josh Peek*
+
 *   Remove old asset_path configuration (no longer needed now that we have the asset pipeline).
 
     *Josh Peek*

guides/source/configuring.md

@@ -567,8 +567,6 @@ Some parts of Rails can also be configured externally by supplying environment v
 
 * `ENV["RAILS_RELATIVE_URL_ROOT"]` is used by the routing code to recognize URLs when you deploy your application to a subdirectory.
 
-* `ENV["RAILS_ASSET_ID"]` will override the default cache-busting timestamps that Rails generates for downloadable assets.
-
 * `ENV["RAILS_CACHE_ID"]` and `ENV["RAILS_APP_VERSION"]` are used to generate expanded cache keys in Rails' caching code. This allows you to have multiple separate caches from the same application.