It is possible to assign a custom memo to individual bookmarks. This can be done when first saving the bookmark, or you can add/edit memos at any time.
Firstly, tap the Bookmarks tab on the tab bar and then on the bookmark you wish to edit.
Tap the Edit icon on top-right tool bar to open the Edit View.
Input a memo in the text box or edit an existing memo.
Tap Done to save your changes.