pidfd_open.2: Add the setns(2) use case for PID file descriptors

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/pidfd_open.2

@@ -178,6 +178,11 @@ The
 .BR pidfd_getfd (2)
 system call can be used to obtain a duplicate of a file descriptor
 of another process referred to by a PID file descriptor.
+.IP \(bu
+A PID file descriptor can be used as the argument of
+.BR setns (2)
+in order to move into one or more of the same namespaces as the process
+referred to by the file descriptor.
 .PP
 The
 .BR pidfd_open ()
@@ -262,5 +267,6 @@ main(int argc, char *argv[])
 .BR pidfd_send_signal (2),
 .BR poll (2),
 .BR select (2),
+.BR setns (2),
 .BR waitid (2),
 .BR epoll (7)