Add Comments field to Designer for each item. Could be for example in Properties - Identification section.
The reasoning is that after some time you will not remember all details why something was done and adding comments help in that. Like in any programming.
I like this concept. I would suggest that rather than being a property of a specific control/element in the interface, the comments could be more like collapsible sticky-notes that can be associated with different parts of the interface (or perhaps just floating with a general connector) since often it is the interaction of the various parts that I find myself forgetting rather than any specific control. Also, that would remove the guessing game of trying to find which control/element had a comment on it 