Clarify ActiveModel::Validator initialization in docs.

As discussed with @josevalim on Ruby Rogues Parley.

activemodel/lib/active_model/validator.rb

@@ -60,6 +60,9 @@ module ActiveModel
   #     end
   #   end
   #
+  # Note that the validator is initialized only once for the whole application
+  # lifecycle, and not on each validation run.
+  #
   # The easiest way to add custom validators for validating individual attributes
   # is with the convenient <tt>ActiveModel::EachValidator</tt>.
   #