Jim, I apologize that you have been unable to find any documentation on sharecropped transactions. I've done some digging and it appears that it was overlooked. I will guarantee you that it will be added in the helps for the next release. In the meantime...
When you apply supplies to a field that is set up as sharecropped, the farming box will ask you for the percent or quantity of supplies that the landlord provided. These will not come out of your inventory, however a record of these will be kept for each field and can be accessed by printing an enterprise statement for "Landlord Inputs Only."
When you harvest, you will be asked for the total harvest as well as the Percent or quantity that is the landlord's. Only your share of the harvest will be added to harvested crop inventory (so you can sell it). As with supplies, a separate record (that does not effect your books) will be kept of the landlord's harvest. This will be included in the enterprise statement for Landlord Inputs Only.
I hope this helps. Again I apologize for this not being in the documentation.