Select the document, and choose Tools > Show Info from the menubar. You’ll see a panel at the bottom of the window that opens, and the panel says “Spotlight Comment”. That’s a good place to enter the info you described.
If you’re using a view (like 3-panes) that has a file list with columns, you can make that Comment visible in one of those columns by choosing View > Columns and selecting “Spotlight Comment”. It’s also possible to click that displayed comment (or blank area) and edit the text there.