net: improve nLastBlockTime and nLastTXTime documentation

Co-authored-by: John Newbery <john@johnnewbery.com>

src/net.cpp

@@ -904,10 +904,10 @@ bool CConnman::AttemptToEvictConnection()
     // Protect the 8 nodes with the lowest minimum ping time.
     // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
     EraseLastKElements(vEvictionCandidates, ReverseCompareNodeMinPingTime, 8);
-    // Protect 4 nodes that most recently sent us transactions.
+    // Protect 4 nodes that most recently sent us novel transactions accepted into our mempool.
     // An attacker cannot manipulate this metric without performing useful work.
     EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
-    // Protect 4 nodes that most recently sent us blocks.
+    // Protect 4 nodes that most recently sent us novel blocks.
     // An attacker cannot manipulate this metric without performing useful work.
     EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);
     // Protect the half of the remaining nodes which have been connected the longest.

src/net.h

@@ -932,8 +932,17 @@ public:
     // Used for headers announcements - unfiltered blocks to relay
     std::vector<uint256> vBlockHashesToAnnounce GUARDED_BY(cs_inventory);
 
-    // Block and TXN accept times
+    /** UNIX epoch time of the last block received from this peer that we had
+     * not yet seen (e.g. not already received from another peer), that passed
+     * preliminary validity checks and was saved to disk, even if we don't
+     * connect the block or it eventually fails connection. Used as an inbound
+     * peer eviction criterium in CConnman::AttemptToEvictConnection. */
     std::atomic<int64_t> nLastBlockTime{0};
+
+    /** UNIX epoch time of the last transaction received from this peer that we
+     * had not yet seen (e.g. not already received from another peer) and that
+     * was accepted into our mempool. Used as an inbound peer eviction criterium
+     * in CConnman::AttemptToEvictConnection. */
     std::atomic<int64_t> nLastTXTime{0};
 
     // Ping time measurement: