What I’d recommend is to go to /admin/customize/site_texts/ search for js.composer.reply_placeholder (or Type here. Use Markdown, BBCode, or HTML to format.) and modify that text to something like
Type here. Use Markdown, BBCode, or HTML to format. Drag or paste images. Use ``` before and after code blocks.
Far too few people will do the tutorial anyway. There is a slight chance that they’ll take notice of that placeholder, but most of them will still just paste the stuff blindly.