githooks documentation: post-checkout hook is also called after clone

The documentation of the post-checkout hook just talks
about git-checkout. But recently git-clone was changed to
call it too, unless the -no-checkout (-n) option is used.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>