Maybe you could think of using
However, comments are anchored to the position of the cursor at time of insertion. If you erase the range containing the anchor, the comment also vanishes. Comments are rather intended for commenting (as their name implies) a sentence or argument when collaboratively editing a document.
With the power of current PCs, your best option is probably;
either to open another document to put down your notes, so that you can easily retrieve them and sort them in a relevant manner,
or to open a small footprint editor like KWrite, gedit, emacs, vi or NotePad and eventually several windows to keep some order and structure in your notes.