iproute2: Document the -D and -I options

While looking into a sysctl regression in decnet on old kernels I
discovered this omission in the iproute2 documentation.

I can't imagine anyone's muscle memory remembering the longer forms.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

ip/ip.c

@@ -48,6 +48,7 @@ static void usage(void)
 "                   netns | l2tp | tcp_metrics }\n"
 "       OPTIONS := { -V[ersion] | -s[tatistics] | -d[etails] | -r[esolve] |\n"
 "                    -f[amily] { inet | inet6 | ipx | dnet | bridge | link } |\n"
+"                    -4 | -6 | -I | -D | -B | -0 |\n"
 "                    -l[oops] { maximum-addr-flush-attempts } |\n"
 "                    -o[neline] | -t[imestamp] | -b[atch] [filename] |\n"
 "                    -rc[vbuf] [size]}\n");

man/man8/ip.8

@@ -81,6 +81,16 @@ shortcut for
 shortcut for
 .BR "\-family bridge" .
 
+.TP
+.B \-D
+shortcut for
+.BR "\-family decnet" .
+
+.TP
+.B \-I
+shortcut for
+.BR "\-family ipx" .
+
 .TP
 .B \-0
 shortcut for