LibreOffice Writer actually already has the feature you want. The marginal notes function in
Writer is simple to use and doesn't require any creation of frames, resizing text, or changing the actual body of the document, so I should experiment with that again as it is ideal for annotating all types of document.
You place your cursor where you want the comment bubble to emerge from (see screenshot below) and either use Insert > Comment or press Ctrl+Alt+C and the comment bubble appears in the margin.
As you can see in the screenshot below, you can have multiple comments and you can use the feature with any other kind of document such as
.txt, but you would have to save it as
.odt to retain the notes. If using the
.doc format, the margin notes will be saved successfully, but use of
.odt is always recommended if possible.
If you want to prevent the comments from displaying go to view > comments and make sure comments is unchecked. When the cursor is inside a comment box in the margin, enter Ctrl+Alt+PgUp to move to the previous comment, and Ctrl+Alt+PgDown to move to the next one.
More information on the comment feature is available at the official site. Full documentation on
LibreOffice and its programs such as
Writer are available in pdf format from this site.
I am reluctant to recommend another text editor, because when
LibreOffice is used with the special comment feature it should actually be what you need for normal plain text documents as well as office documents.