ipv4: remove is_data also from ip_options documentation.

commit ef722495
( [IPV4]: Remove unused ip_options->is_data) removed the unused is_data
member from ip_options struct.

This patch removes is_data also from the documentation of the ip_options struct.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>