docs, nodoc `NullPreloader` and `AlreadyLoaded`.

These classes are part of Active Record Preloader, which is not part of
the public API.

activerecord/lib/active_record/associations/preloader.rb

@@ -160,7 +160,7 @@ def grouped_records(association, records)
         h
       end
 
-      class AlreadyLoaded
+      class AlreadyLoaded # :nodoc:
         attr_reader :owners, :reflection
 
         def initialize(klass, owners, reflection, preload_scope)
@@ -175,7 +175,7 @@ def preloaded_records
         end
       end
 
-      class NullPreloader
+      class NullPreloader # :nodoc:
         def self.new(klass, owners, reflection, preload_scope); self; end
         def self.run(preloader); end
         def self.preloaded_records; []; end