jail.2: Mention EPERM is returned on open directories

The manual page does not directly mention this reason for getting EPERM,
instead referring the reader to chroot(2). We have had some questions
about it recently, in part due to a bug (fixed), and this case is not an
obvious permission/problem, so let's be more explicit.

PR: 280809
Reviewed by: jamie
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52319