switch: mention the --detach option when dying due to lack of a branch

switch: mention the --detach option when dying due to lack of a branch

Users who are accustomed to doing `git checkout <tag>` assume that
`git switch <tag>` will do the same thing. Inform them of the --detach
option so they aren't left wondering why `git switch` doesn't work but
`git checkout` does.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>