man: describe --bpf option of ss

This option has been recently added to ss utility.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>

man/man8/ss.8

@@ -57,6 +57,9 @@ Print summary statistics. This option does not parse socket lists obtaining
 summary from various sources. It is useful when amount of sockets is so huge
 that parsing /proc/net/tcp is painful.
 .TP
+.B \-b, \-\-bpf
+Show socket BPF filters (only administrators are allowed to get these information).
+.TP
 .B \-4, \-\-ipv4
 Display only IP version 4 sockets (alias for -f inet).
 .TP