net/core: Document __skb_flow_dissect() flags argument

This patch avoids that the following warning is reported when building
with W=1:

warning: Function parameter or member 'flags' not described in '__skb_flow_dissect'

Cc: Tom Herbert <tom@herbertland.com>
Fixes: cd79a238 ("flow_dissector: Add flags argument to skb_flow_dissector functions") # v4.3.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David S. Miller <davem@davemloft.net>