create a procedure based on column data change in postgrersql

begin;
create temp table account_details(email text primary key, username text, password text);
insert into account_details(email, username, password) values('a.com','b','c'),('b.com','d','e');
commit;

CREATE TRIGGER trigger_update_account_details
AFTER UPDATE ON account_details
FOR EACH ROW
WHEN (OLD.email    IS DISTINCT FROM NEW.email
   OR OLD.username IS DISTINCT FROM NEW.username
   OR OLD.password IS DISTINCT FROM NEW.password)
EXECUTE PROCEDURE notify_insert_account_details();
________________________________________________________________________

What possible  PROCEDURE notify_insert_account_details() can be. 
The following is my immature test code. 
CREATE OR REPLACE FUNCTION notify_insert_account_details()
  RETURNS trigger
  LANGUAGE plpgsql AS
$$
BEGIN
RAISE NOTICE 'hello world';
END
$$;
_______________________________________________________
when I update then error occurs. the update clause didn't execute. the function did fired. 
NOTICE:  hello world
ERROR:  control reached end of trigger procedure without RETURN
CONTEXT:  PL/pgSQL function notify_insert_account_details()
______________________________________________
I am wondering how sophisticated this procedure/function notify_insert_account_details() can become, let's say linked within 3 table. 
Can anyone showcase an example/demo? 

I also asked same question on stackoverflow. Since it's more easy to search.  (https://stackoverflow.com/questions/69631945/create-a-procedure-based-on-column-data-change-in-postgrersql),