Document the fact that the firewall allows pings by default in rl-1603.xml.

nixos/doc/manual/release-notes/rl-1603.xml

@@ -262,6 +262,18 @@ services.syncthing = {
     </programlisting>
   </listitem>
 
+  <listitem>
+    <para>
+      <literal>networking.firewall.allowPing</literal> is now enabled by
+      default. Users are encourarged to configure an approiate rate limit for
+      their machines using the Kernel interface at
+      <filename>/proc/sys/net/ipv4/icmp_ratelimit</filename> and
+      <filename>/proc/sys/net/ipv6/icmp/ratelimit</filename> or using the
+      firewall itself, i.e. by setting the NixOS option
+      <literal>networking.firewall.pingLimit</literal>.
+    </para>
+  </listitem>
+
 </itemizedlist>