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>

Documentation/git-log.txt

@@ -39,7 +39,9 @@ OPTIONS
 	full ref name (including prefix) will be printed. If 'auto' is
 	specified, then if the output is going to a terminal, the ref names
 	are shown as if 'short' were given, otherwise no ref names are
-	shown. The default option is 'short'.
+	shown. The option `--decorate` is short-hand for `--decorate=short`.
+	Default to configuration value of `log.decorate` if configured,
+	otherwise, `auto`.
 
 --decorate-refs=<pattern>::
 --decorate-refs-exclude=<pattern>::