[doc] assert that default min relay feerate and incremental are the same

src/node/mempool_args.cpp

@@ -65,6 +65,7 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& argsman, const CChainP
         }
     }
 
+    static_assert(DEFAULT_MIN_RELAY_TX_FEE == DEFAULT_INCREMENTAL_RELAY_FEE);
     if (const auto arg{argsman.GetArg("-minrelaytxfee")}) {
         if (std::optional<CAmount> min_relay_feerate = ParseMoney(*arg)) {
             // High fee check is done afterward in CWallet::Create()