doc: warn that CheckBlock() underestimates sigops

Counting sigops in the witness and for p2sh requires
context that CheckBlock()  does not have, so it only
counts a subset of sigops.

The check here was introduced by Satoshi as a "cleanup" in
f1e1fb4bde. With the attempted
introduction of OP_EVAL, it was replaced by the check in
ConnectBlock(). Commit e679ec969c
marked this code as a placeholder for backward compatibility.

Then when P2SH replaced OP_EVAL in 922e8e2929
the phrase "compatibility-breaking" was replaced by a simple
observation that before v0.6 this is how sigops were counted.

It's unclear why the check was kept and there were no review comments
about it.

src/validation.cpp

@@ -4094,6 +4094,8 @@ 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 witness and p2sh sigops.
     unsigned int nSigOps = 0;
     for (const auto& tx : block.vtx)
     {