assumeutxo: Add documentation on dumptxoutset serialization format

src/rpc/blockchain.cpp

@@ -2676,6 +2676,11 @@ UniValue CreateUTXOSnapshot(
     unsigned int iter{0};
     std::vector<std::pair<uint32_t, Coin>> coins;
 
+    // In the following code we collect all the coins (in the coins vector)
+    // that belong to a certain txid (key.hash) and when we have them all
+    // (key.hash != last_hash) we write them to file using the below lamda
+    // function.
+    // See also https://github.com/bitcoin/bitcoin/issues/25675
     auto write_coins_to_file = [&](AutoFile& afile, const uint256& last_hash, const std::vector<std::pair<uint32_t, Coin>>& coins) {
         afile << last_hash;
         WriteCompactSize(afile, coins.size());