Provide missing kdoc of fields of struct tcf_pkt_info and tcf_ematch_ops.

Found using ./scripts/kernel-doc -none -Werror include/net/pkt_cls.h
Signed-off-by: Bijie Xu <bijie.xu@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>