|| versus concat( ), diff behavior

Can anybody please point me to where this "difference of behavior" is explained/documented ?
Thanks,
-dvs-

-- version = 9.1.3
do $$
declare
v_str char(10);
begin
v_str := 'abc' ;
raise info '%', concat(v_str, v_str) ;
raise info '%', v_str||v_str ;
end
$$;

INFO: abc abc
INFO: abcabc