doc: add "git switch -c" as another option on detached HEAD

doc: add "git switch -c" as another option on detached HEAD

In the "DETACHED HEAD" section in the git-checkout doc, it suggests
using "git checkout -b <branch-name>" to create a new branch on the
detached head.

On the other hand, when you checkout a commit that is not at the tip of
any named branch (e.g., when you checkout a tag), git suggests using
"git switch -c <branch-name>".

Add "git switch -c" as another option and mitigate this inconsistency.

Signed-off-by: Yutaro Ohno <yutaro.ono.418@gmail.com>
Acked-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>