doc: git-checkout: deduplicate --detach explanation

doc: git-checkout: deduplicate --detach explanation

From user feedback: several users say they don't understand the use case
for `--detach`. It's probably not realistic to explain the use case for
detached HEAD state here, but we can improve the situation.

Explain how `git checkout --detach` is different from
`git checkout <branch>` instead of copying over the description from
`git checkout <branch>`, since `git checkout <branch>` will be a
familiar command to many readers.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>