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: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>