Consider Including a Warning for Bitcoin's Merkle Tree

[tommagnusson]

Location: https://bitcoin.org/en/developer-guide#transaction-data

In the source code for bitcoin's merkle tree implementation there exists a large warning comment explaining a security vulnerability: https://github.com/bitcoin/bitcoin/blob/master/src/consensus/merkle.cpp#L22

Perhaps consider adding a similar warning in the documentation on the website, because it appears to be a naive way of constructing a merkle tree.

[harding]

@tommagnusson I think we already have a warning in that section:

I admit, that's a bit wordy and could be improved.