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

lib/libsys/jail.2

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 9, 2025
+.Dd September 15, 2025
 .Dt JAIL 2
 .Os
 .Sh NAME
@@ -555,6 +555,9 @@ and
 call
 .Xr chroot 2
 internally, so they can fail for all the same reasons.
+In particular, they return the
+.Bq Er EPERM
+error when the process to join a jail has open directories.
 Please consult the
 .Xr chroot 2
 manual page for details.