Document that load hooks run if already triggered

After first learning of this in c2a3ff0 and later discussion about cross
component dependencies, it makes sense to document it explicitly.

activesupport/lib/active_support/lazy_load_hooks.rb

@@ -26,6 +26,18 @@ module ActiveSupport
   # run_load_hooks will then execute all the hooks that were registered
   # with the on_load method. In the case of the above example, it will
   # execute the block of code that is in the +initializer+.
+  #
+  # Registering a hook that has already run results in that hook executing
+  # immediately. This allows hooks to be nested for code that relies on
+  # multiple lazily loaded components:
+  #
+  #   initializer "action_text.renderer" do
+  #     ActiveSupport.on_load(:action_controller_base) do
+  #       ActiveSupport.on_load(:action_text_content) do
+  #         self.default_renderer = Class.new(ActionController::Base).renderer
+  #       end
+  #     end
+  #   end
   module LazyLoadHooks
     def self.extended(base) # :nodoc:
       base.class_eval do
@@ -36,7 +48,8 @@ module ActiveSupport
     end
 
     # Declares a block that will be executed when a Rails component is fully
-    # loaded.
+    # loaded. If the component has already loaded, the block is executed
+    # immediately.
     #
     # Options:
     #