[docs] Improve commenting in ProcessGetData()

Github-Pull: #18808
Rebased-From: 9847e205bf

src/net_processing.cpp

@@ -1573,10 +1573,14 @@ void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnm
     {
         LOCK(cs_main);
 
+        // Process as many TX items from the front of the getdata queue as
+        // possible, since they're common and it's efficient to batch process
+        // them.
         while (it != pfrom->vRecvGetData.end() && (it->type == MSG_TX || it->type == MSG_WITNESS_TX)) {
             if (interruptMsgProc)
                 return;
-            // Don't bother if send buffer is too full to respond anyway
+            // The send buffer provides backpressure. If there's no space in
+            // the buffer, pause processing until the next call.
             if (pfrom->fPauseSend)
                 break;
 
@@ -1613,6 +1617,8 @@ void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnm
         }
     } // release cs_main
 
+    // Only process one BLOCK item per call, since they're uncommon and can be
+    // expensive to process.
     if (it != pfrom->vRecvGetData.end() && !pfrom->fPauseSend) {
         const CInv &inv = *it++;
         if (inv.type == MSG_BLOCK || inv.type == MSG_FILTERED_BLOCK || inv.type == MSG_CMPCT_BLOCK || inv.type == MSG_WITNESS_BLOCK) {