man/ping: Document CAP_NET_ADMIN for -m

Link: iputils#515
Reviewed-by: Matthias Gerstner <matthias.gerstner@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

doc/ping.xml

@@ -357,7 +357,12 @@ xml:id="man.ping">
           <emphasis remap="I">mark</emphasis> to tag the packets
           going out. This is useful for variety of reasons within
           the kernel such as using policy routing to select
-          specific outbound processing.</para>
+          specific outbound processing.
+          CAP_NET_ADMIN capability is required, see
+          <citerefentry>
+            <refentrytitle>socket</refentrytitle>
+            <manvolnum>7</manvolnum>
+          </citerefentry>.</para>
         </listitem>
       </varlistentry>
       <varlistentry>