netfilter: nf_tables: uapi: Describe NFTA_RULE_CHAIN_ID

Add a brief description to the enum's comment.

Fixes: 837830a4 ("netfilter: nf_tables: add NFTA_RULE_CHAIN_ID attribute")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>