ipvs: Update CONFIG_IP_VS_IPV6 description and help text

This adds a URL to further info to the CONFIG_IP_VS_IPV6 Kconfig help
text. Also, I think it should be ok to remove the "DANGEROUS" label in the
description line at this point to get people to try it out and find all
the bugs ;) It's still marked as experimental, of course.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>