Expand ActionDispatch::Reloader docs

actionpack/lib/action_dispatch/middleware/reloader.rb

 module ActionDispatch
-  # ActionDispatch::Reloader provides to_prepare and to_cleanup callbacks.
-  # These are analogs of ActionDispatch::Callback's before and after
-  # callbacks, with the difference that to_cleanup is not called until the
+  # ActionDispatch::Reloader provides prepare and cleanup callbacks,
+  # intended to assist with code reloading during development.
+  #
+  # Prepare callbacks are run before each request, and cleanup callbacks
+  # after each request. In this respect they are analogs of ActionDispatch::Callback's
+  # before and after callbacks. However, cleanup callbacks are not called until the
   # request is fully complete -- that is, after #close has been called on
   # the request body. This is important for streaming responses such as the
   # following:
@@ -15,7 +18,10 @@ module ActionDispatch
   # classes before they are unloaded.
   #
   # By default, ActionDispatch::Reloader is included in the middleware stack
-  # only in the development environment.
+  # only in the development environment; specifically, when config.cache_classes
+  # is false. Callbacks may be registered even when it is not included in the
+  # middleware stack, but are executed only when +ActionDispatch::Reloader.prepare!+
+  # or +ActionDispatch::Reloader.cleanup!+ are called manually.
   #
   class Reloader
     include ActiveSupport::Callbacks
@@ -23,8 +29,8 @@ class Reloader
     define_callbacks :prepare, :scope => :name
     define_callbacks :cleanup, :scope => :name
 
-    # Add a preparation callback. Preparation callbacks are run before each
-    # request.
+    # Add a prepare callback. Prepare callbacks are run before each request, prior
+    # to ActionDispatch::Callback's before callbacks.
     def self.to_prepare(*args, &block)
       set_callback(:prepare, *args, &block)
     end
@@ -35,10 +41,12 @@ def self.to_cleanup(*args, &block)
       set_callback(:cleanup, *args, &block)
     end
 
+    # Execute all prepare callbacks.
     def self.prepare!
       new(nil).send(:_run_prepare_callbacks)
     end
 
+    # Execute all cleanup callbacks.
     def self.cleanup!
       new(nil).send(:_run_cleanup_callbacks)
     end