document: donation field in tx

[ashisherc]

It will be helpful to include donation field information in the Conway Ledger Spec. eg. A tx must include enough inputs to cover the donation amount, and according to the figure 43, the donations are not processed (treasury updated) until the epoch transition.

[carlostome]

Hi,
this will be addressed by #662, which is still on the making.
Take a look at the changes there regarding txdonation and if you feel something is still missing, feedback is very welcomed!