netfilter: Correct documentation errors in nf_tables.h

NFTA_RANGE_OP incorrectly says nft_cmp_ops instead of nft_range_ops.
NFTA_LOG_GROUP and NFTA_LOG_QTHRESHOLD claim NLA_U32 instead of NLA_U16
NFTA_EXTHDR_SREG isn't documented as a register

Signed-off-by: Matthieu De Beule <matthieu.debeule@proton.ch>
Signed-off-by: Florian Westphal <fw@strlen.de>