net: fix documentation of skb_needs_linearize().

skb_needs_linearize() does not check highmem DMA as it does not call
illegal_highdma() anymore, so there is no need to mention highmem DMA here.

(Indeed, ~NETIF_F_SG flag, which is checked in skb_needs_linearize(), can
be set when illegal_highdma() returns true, and we are assured that
illegal_highdma() is invoked prior to skb_needs_linearize() as
skb_needs_linearize() is a static method called only once.
But ~NETIF_F_SG can be set not only there in this same invocation path.
It can also be set when can_checksum_protocol() returns false).

see commit 02932ce9,
Convert skb_need_linearize() to use precomputed features.
Signed-off-by: Rami Rosen <rosenr@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>