luci-mod-network: Add help text for many existing odhcpd IPv6 parameters

and Global network options ULA.

Relocated the "DHCPv6 Mode" to below RA-Service: DHCPv6-Mode is actually
ra_management in disguise. Better grouping.

Until now, some of the IPv6 settings are mysterious. They are now
clarified. The information is accessible to IPv6 beginners.

Signed-off-by: Paul Dee <itsascambutmailmeanyway@gmail.com>