doc: warn that CheckBlock() underestimates sigops

Counting sigops in the witness requires context that CheckBlock()  does not have, so it only counts sigops for non-segwit transactions.

This should not be a problem. There are 2 call sites:

1. ConnectBlock(): this checks all sigops
2. TestBlockValidity() used by getblocktemplate: this also calls ConnectBlock()
3. AcceptBlock(): this performs limited checks before storing a block on disk
4. ProcessNewBlock(): calls AcceptBlock()
4. VerifyDB(): disconnects the block at level 3, which results in ConnectBlock()

src/validation.cpp

@@ -4047,6 +4047,7 @@ bool CheckBlock(const CBlock& block, BlockValidationState& state, const Consensu
                                  strprintf("Transaction check failed (tx hash %s) %s", tx->GetHash().ToString(), tx_state.GetDebugMessage()));
         }
     }
+    // This underestimates the number of sigops, because unlike ConnectBlock it does not count the witness:
     unsigned int nSigOps = 0;
     for (const auto& tx : block.vtx)
     {