[trivial] Add comment documenting bumpfee mapValues

src/wallet/wallet.h

@@ -256,6 +256,10 @@ public:
      *
      *     "comment", "to"   - comment strings provided to sendtoaddress,
      *                         sendfrom, sendmany wallet RPCs
+     *     "replaces_txid"   - txid (as HexStr) of transaction replaced by
+     *                         bumpfee on transaction created by bumpfee
+     *     "replaced_by_txid" - txid (as HexStr) of transaction created by
+     *                         bumpfee on transaction replaced by bumpfee
      *     "from", "message" - obsolete fields that could be set in UI prior to
      *                         2011 (removed in commit 4d9b223)
      *