Hey Kate,
Person Notes doesn't push through...randomly enough you can't send the data into the description field. Best thing to do is to just create a new custom field for description OR a supplementary field where you can store this data.
Create it on the Lead and Contact record and map the two before it comes over to Marketo. Only thing that's a bit tough on this field is Marketo doesn't have a concatenation feature now so you'd be updating the field everytime someone submitted new information. Best way to take care of this is to ALSO submit the comment into a logged call on the Lead or Contact record in the comments field.