Improve comment

Provide a more accurate description of the behavior we are avoiding and
why the current behavior is used.

[ci skip]

Author: cupakromer

lib/rspec-rails.rb

@@ -52,17 +52,17 @@ def config_default_preview_path(options)
       end
 
       def supports_action_mailer_previews?(config)
-        # If the action mailer railtie isn't loaded the config will not
-        # respond.
+        # These checks avoid loading `ActionMailer`. Using `defined?` has the
+        # side-effect of the class getting loaded if it is available. This is
+        # problematic because loading `ActionMailer::Base` will cause it to
+        # read the config settings; this is the only time the config is read.
+        # If the config is loaded now, any settings declared in a config block
+        # in an initializer will be ignored.
         #
-        # This string version check avoids loading the ActionMailer class, as
-        # would happen using `defined?`. This is necessary because the
-        # ActionMailer class only loads it's settings once, at load time. If we
-        # load the class now any settings declared in a config block in an
-        # initializer will be ignored.
-        #
-        # We cannot use `config.action_mailer.respond_to?(:preview_path)` here
-        # as it will always return `true`.
+        # If the action mailer railtie has not been loaded then `config` will
+        # not respond to the method. However, we cannot use
+        # `config.action_mailer.respond_to?(:preview_path)` here as it will
+        # always return `true`.
         config.respond_to?(:action_mailer) && ::Rails::VERSION::STRING > '4.1'
       end
     end