Mention that Action Mailer methods are not deferred [ci skip]

Closes #17965, #17968

[Godfrey Chan, Miklós Fazekas]

guides/source/upgrading_ruby_on_rails.md

@@ -207,12 +207,47 @@ gem 'rails-deprecated_sanitizer'
 ```
 
 ### Rails DOM Testing
+
 The [`TagAssertions` module](http://api.rubyonrails.org/classes/ActionDispatch/Assertions/TagAssertions.html) (containing methods such as `assert_tag`), [has been deprecated](https://github.com/rails/rails/blob/6061472b8c310158a2a2e8e9a6b81a1aef6b60fe/actionpack/lib/action_dispatch/testing/assertions/dom.rb) in favor of the `assert_select` methods from the `SelectorAssertions` module, which has been extracted into the [rails-dom-testing gem](https://github.com/rails/rails-dom-testing).
 
 
 ### Masked Authenticity Tokens
+
 In order to mitigate SSL attacks, `form_authenticity_token` is now masked so that it varies with each request.  Thus, tokens are validated by unmasking and then decrypting.  As a result, any strategies for verifying requests from non-rails forms that relied on a static session CSRF token have to take this into account.
 
+### Action Mailer
+
+Previously, calling a mailer method on a mailer class will result in the
+corresponding instance method being executed directly. With the introduction of
+Active Job and `#deliver_later`, this is no longer true. In Rails 4.2, the
+invocation of the instance methods are deferred until either `deliver_now` or
+`deliver_later` is called. For example:
+
+```ruby
+class Notifier < ActionMailer::Base
+  def notify(user, ...)
+    puts "Called"
+    mail(to: user.email, ...)
+  end
+end
+
+mail = Notifier.notify(user, ...) # Notifier#welcome is not yet called at this point
+mail = mail.deliver_now           # Prints "Called"
+```
+
+This should not result in any noticible differnces for most applications.
+However, if you need some non-mailer methods to be exectuted synchronously, and
+you were previously relying on the synchronous proxying behavior, you should
+define them as class methods on the mailer class directly:
+
+```ruby
+class Notifier < ActionMailer::Base
+  def self.broadcast_notifications(users, ...)
+    users.each { |user| Notifier.notify(user, ...) }
+  end
+end
+```
+
 Upgrading from Rails 4.0 to Rails 4.1
 -------------------------------------