We can add notes to features by selecting the notes column in the feature grid. If the notes column is not available click on Columns Shown and select the Notes field.

But we do not have the option to edit feature notes in the feature backlog and the notes field is not available to view/edit in the program board.

Similar to the features grid it will be good to have the option to edit the feature notes field in the feature backlog and then view/edit option in the program board.