43.10 Trigger Functions one sentences kind of not easy to understand.

Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value. Returning a row value different from the original value of NEW alters the row that will be inserted or updated. Thus, if the trigger function wants the triggering action to succeed normally without altering the row value, NEW (or a value equal thereto) has to be returned. To alter the row to be stored, it is possible to replace single values directly in NEW and return the modified NEW, or to build a complete new record/row to return. In the case of a before-trigger on DELETE, the returned value has no direct effect, but it has to be nonnull to allow the trigger action to proceed. Note that NEW is null in DELETE triggers, so returning that is usually not sensible. The usual idiom in DELETE triggers is to return OLD.

begin;
CREATE TABLE documents (
    docdesc text,
    misc text,
    modification_time timestamp with time zone DEFAULT now()
);
CREATE FUNCTION documents_update_mod_time() RETURNS trigger
AS $$
    begin
    new.modification_time := now(); 

    return new;

    end
$$
    LANGUAGE plpgsql;
CREATE TRIGGER documents_modification_time
    BEFORE INSERT OR UPDATE ON documents
    FOR EACH ROW
    EXECUTE PROCEDURE documents_update_mod_time();
commit;