As KVogler said, comment nodes do not need to be attached to any specific thing and can be used for exactly that.
You can press C while the mouse is over an empty space in the graph to create a new comment (or right click and use Add Comment…). Pressing C while nodes are selected wraps them in a comment as well.
You can edit the color or use whatever search tags you want. For example, if you start your comments with “TODO:”, you can use Ctrl+F to find all references to TODO in the Blueprint or use Ctrl+Shift+F to search all Blueprints in your game in order to make a TODO list.
Cheers,
Michael Noland