Using of the term `entropy` is unconsistent and misleading

Functions like `wally_tx_add_elements_raw_input` are expecting `entropy` as an argument. The doc says that this is "Entropy for the asset tag calculation."

The man for `wally_tx_elements_issuance_generate_entropy` says "Calculate the asset entropy from a prevout and the Ricardian contract hash."

Actually `entropy` in `wally_tx_add_elements_raw_input` is `contract_hash` argument to `wally_tx_elements_issuance_generate_entropy`.

I think this is all very confusing, and I suggest at least to mention explicitly what is expected as `entropy` [here](https://github.com/ElementsProject/libwally-core/blob/46da3615b8e0837964f443d6b113ad500ea89f97/include/wally_transaction.h#L947).

Or maybe clarify variable names.