man: ip: add -h[uman-readable] option

Since 'ip help' lists it, it should be described in ip.8 as well.
Signed-off-by: Phil Sutter <phil@nwl.cc>

man/man8/ip.8

@@ -27,6 +27,7 @@ ip \- show / manipulate routing, devices, policy routing and tunnels
 .ti -8
 .IR OPTIONS " := { "
 \fB\-V\fR[\fIersion\fR] |
+\fB\-h\fR[\fIuman-readable\fR] |
 \fB\-s\fR[\fItatistics\fR] |
 \fB\-r\fR[\fIesolve\fR] |
 \fB\-f\fR[\fIamily\fR] {
@@ -45,6 +46,10 @@ Print the version of the
 .B ip
 utility and exit.
 
+.TP
+.BR "\-h", " \-human", " \-human-readable"
+output statistics with human readable values followed by suffix.
+
 .TP
 .BR "\-b", " \-batch " <FILENAME>
 Read commands from provided file or standard input and invoke them.