Be a bit clearer in the new-adapter-API argument error

Author: matthewd

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -100,7 +100,7 @@ def initialize(config_or_deprecated_connection, deprecated_logger = nil, depreca
           @logger = ActiveRecord::Base.logger
 
           if deprecated_logger || deprecated_connection_options || deprecated_config
-            raise ArgumentError, "new API accepts just one config hash"
+            raise ArgumentError, "when initializing an ActiveRecord adapter with a config hash, that should be the only argument"
           end
         else
           # Soft-deprecated for now; we'll probably warn in future.