Fix typo in CHANGES
Improve documentation of 'fold', and exceptions thrown

In retrospect,  I think I would opt to not include the transactionMode
field of the FoldOptions type,  and just require that people create
their own transaction if they want something other than the default.
Also,  I think I would probably chosen REPEATABLE READ as the default
transaction isolation level if the fold operator creates its own transaction.

Making these changes would likely be fairly easy for most people,  but
even so I'm not sure if there's enough justification for it.

Also,  I should probably change the exceptions thrown from the toField
definition for Values.   FormatError might make sense,  however there
isn't really the right information available to fill out all the FormatError
fields.   So we could choose a different exception,  or perhaps we could
add an error case to the Action data type.