clarify processing of mempool-msgs when NODE_BLOOM

Under which circumstances we process received 'mempool' P2P messages
caused confusion in bitcoin#27426. Rather than bikeshedding the formulation
of the IF-statement, this adds a comment clarifing when we process
the message. Also, correcting the comment of `m_send_mempool`.

Co-authored-by: willcl-ark <will8clark@gmail.com>

src/net_processing.cpp

@@ -292,7 +292,8 @@ struct Peer {
          *  this does not have to be sorted. */
         std::set<uint256> m_tx_inventory_to_send GUARDED_BY(m_tx_inventory_mutex);
         /** Whether the peer has requested us to send our complete mempool. Only
-         *  permitted if the peer has NetPermissionFlags::Mempool. See BIP35. */
+         *  permitted if the peer has NetPermissionFlags::Mempool or we advertise
+         *  NODE_BLOOM. See BIP35. */
         bool m_send_mempool GUARDED_BY(m_tx_inventory_mutex){false};
         /** The last time a BIP35 `mempool` request was serviced. */
         std::atomic<std::chrono::seconds> m_last_mempool_req{0s};
@@ -4600,6 +4601,8 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
     }
 
     if (msg_type == NetMsgType::MEMPOOL) {
+        // Only process received mempool messages if we advertise NODE_BLOOM
+        // or if the peer has mempool permissions.
         if (!(peer->m_our_services & NODE_BLOOM) && !pfrom.HasPermission(NetPermissionFlags::Mempool))
         {
             if (!pfrom.HasPermission(NetPermissionFlags::NoBan))