I'm afraid to disappoint you but there is no txt help file, especially ascii. Although there is html help file, you may open it through context menu. Please look at the screenshot one more time: https://addons.mozilla.org/en-US/thunderbird/addon/messagenotesplus/ .
Another thing is that help is not quite detailed. But nobody requests to improve it. Be the first one :) Just let me know.
After you click on 'Memorize note', you may use 'Add Memorized note' to add the same note to other messages. Also you may 'Add Memorized note' while multiple messages selected. The note will be added to all of them.