netfilter: nf_tables: add missing descriptions in nft_ct_keys

We missed to add descriptions about NFT_CT_LABELS, NFT_CT_PKTS and
NFT_CT_BYTES, now add it.
Signed-off-by: NLiping Zhang <zlpnobody@gmail.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -860,6 +860,9 @@ enum nft_rt_attributes {
  * @NFT_CT_PROTOCOL: conntrack layer 4 protocol
  * @NFT_CT_PROTO_SRC: conntrack layer 4 protocol source
  * @NFT_CT_PROTO_DST: conntrack layer 4 protocol destination
+ * @NFT_CT_LABELS: conntrack labels
+ * @NFT_CT_PKTS: conntrack packets
+ * @NFT_CT_BYTES: conntrack bytes
  */
 enum nft_ct_keys {
 	NFT_CT_STATE,