update Active Job default adapter [ci skip]

Follow up to 625baa69

guides/source/action_mailer_basics.md

@@ -204,10 +204,14 @@ class UsersController < ApplicationController
 end
 ```
 
-NOTE: Active Job's default behavior is to execute jobs ':inline'. So, you can use
-`deliver_later` now to send emails, and when you later decide to start sending
-them from a background job, you'll only need to set up Active Job to use a queueing
-backend (Sidekiq, Resque, etc).
+NOTE: Active Job's default behavior is to execute jobs via the `:async` adapter. So, you can use
+`deliver_later` now to send emails asynchronously.
+Active Job's default adapter runs jobs with an in-process thread pool.
+It's well-suited for the development/test environments, since it doesn't require
+any external infrastructure, but it's a poor fit for production since it drops
+pending jobs on restart.
+If you need a persistent backend, you will need to use an Active Job adapter
+that has a persistent backend (Sidekiq, Resque, etc).
 
 If you want to send emails right away (from a cronjob for example) just call
 `deliver_now`:

guides/source/configuring.md

@@ -583,7 +583,7 @@ There are a few configuration options available in Active Support:
 
 `config.active_job` provides the following configuration options:
 
-* `config.active_job.queue_adapter` sets the adapter for the queueing backend. The default adapter is `:inline` which will perform jobs immediately. For an up-to-date list of built-in adapters see the [ActiveJob::QueueAdapters API documentation](http://api.rubyonrails.org/classes/ActiveJob/QueueAdapters.html).
+* `config.active_job.queue_adapter` sets the adapter for the queueing backend. The default adapter is `:async`. For an up-to-date list of built-in adapters see the [ActiveJob::QueueAdapters API documentation](http://api.rubyonrails.org/classes/ActiveJob/QueueAdapters.html).
 
     ```ruby
     # Be sure to have the adapter's gem in your Gemfile