Comment clarifications in coins.cpp

src/coins.h

@@ -297,6 +297,9 @@ private:
 void AddCoins(CCoinsViewCache& cache, const CTransaction& tx, int nHeight);
 
 //! Utility function to find any unspent output with a given txid.
+// This function can be quite expensive because in the event of a transaction
+// which is not found in the cache, it can cause up to MAX_OUTPUTS_PER_BLOCK
+// lookups to database, so it should be used with care.
 const Coin& AccessByTxid(const CCoinsViewCache& cache, const uint256& txid);
 
 #endif // BITCOIN_COINS_H