CreateTransaction documentation should be improved

[rrw-zilliqa]

I recently had cause to implement a call to CreateTransaction from scratch and it would be nice to improve the docs:

• CreateTransaction isn't clear on which fields are mandatory, or on how the signature is computed.
• There is various validation (at least in zq1) on eg. address checksums that is not disclosed in the docs

I had to look at the C++ in ZQ1 to get ZQ1 to accept my txn and I shouldn't've ...