netfilter: nf_tables: Add missing Kernel doc

- Add missing documentation of struct field and enum items.
- Add missing documentation of function parameter.

Flagged by ./scripts/kernel-doc -none.

No functional change intended.
Compile tested only.

Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>