net: add doc for in6_pton()

It is not easy to use in6_pton() correctly without reading
its definition, so add some doc for it.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>