doc/log: correct default for --decorate

doc/log: correct default for --decorate

There're two different default options for log --decorate:
* Should `--decorate` be given without any arguments, it's default to
  `short`
* Should neither `--decorate` nor `--no-decorate` be given, it's default
  to the `log.decorate` or `auto`.

We documented the former, but not the latter.

Let's document them, too.

Reported-by: Andy AO <zen96285@gmail.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>