Adjust documentation for IPv6 set up

Signed-off-by: Egor Savkin <es@m-labs.hk>

doc/manual/installing.rst

@@ -287,9 +287,9 @@ and then reboot the device (with ``artiq_flash start`` or a power cycle).
 If the ip config field is not set, or set to "use_dhcp" then the device will
 attempt to obtain an IP address and default gateway using DHCP. If a static IP
 address is wanted, install OpenOCD as before, and flash the IP, default gateway
-(and, if necessary, MAC) addresses directly: ::
+(and, if necessary, MAC and IPv6) addresses directly: ::
 
-  $ artiq_mkfs flash_storage.img -s mac xx:xx:xx:xx:xx:xx -s ip xx.xx.xx.xx/xx -s ipv4_default_route xx.xx.xx.xx
+  $ artiq_mkfs flash_storage.img -s mac xx:xx:xx:xx:xx:xx -s ip xx.xx.xx.xx/xx -s ipv4_default_route xx.xx.xx.xx -s ip6 xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx/xx -s ipv6_default_route xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx
   $ artiq_flash -t [board] -V [variant] -f flash_storage.img storage start
 
 For Kasli devices, flashing a MAC address is not necessary as they can obtain it from their EEPROM.
@@ -307,6 +307,8 @@ Check that you can ping the device. If ping fails, check that the Ethernet link
 
 If you want to use IPv6, the device also has a link-local address that corresponds to its EUI-64, and an additional arbitrary IPv6 address can be defined by using the ``ip6`` configuration key. All IPv4 and IPv6 addresses can be used at the same time.
 
+If IPv6 ping fails, check that the IPv6 default route is set up correctly. In general, the default route should point to the global unicast address of the next-hop gateway/router.
+
 Miscellaneous configuration of the core device
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