Add comment explaining intended use of m_tx_relay

src/net.h

@@ -733,6 +733,7 @@ public:
         int64_t nextSendTimeFeeFilter{0};
     };
 
+    // m_tx_relay == nullptr if we're not relaying transactions with this peer
     std::unique_ptr<TxRelay> m_tx_relay;
     // Used for headers announcements - unfiltered blocks to relay
     std::vector<uint256> vBlockHashesToAnnounce GUARDED_BY(cs_inventory);