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>

include/uapi/linux/netfilter/nf_tables.h

@@ -263,6 +263,7 @@ enum nft_chain_attributes {
  * @NFTA_RULE_USERDATA: user data (NLA_BINARY, NFT_USERDATA_MAXLEN)
  * @NFTA_RULE_ID: uniquely identifies a rule in a transaction (NLA_U32)
  * @NFTA_RULE_POSITION_ID: transaction unique identifier of the previous rule (NLA_U32)
+ * @NFTA_RULE_CHAIN_ID: add the rule to chain by ID, alternative to @NFTA_RULE_CHAIN (NLA_U32)
  */
 enum nft_rule_attributes {
 	NFTA_RULE_UNSPEC,