git-checkout.txt: document -d short option for --detach

`git checkout` learned -d as short option for --detach in 163e3b2975
(switch: add short option for --detach, 2019-03-29) but the
documentation was never updated to reflect the change.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -198,6 +198,7 @@ Use `--no-guess` to disable this.
 	Create the new branch's reflog; see linkgit:git-branch[1] for
 	details.
 
+-d::
 --detach::
 	Rather than checking out a branch to work on it, check out a
 	commit for inspection and discardable experiments.