The markdown spellchecker is not really meant for new users. Unfortunately, the markdown editor component that Joplin uses (CodeMirror) doesn't support native spellchecking, in fact, none of the popular javascript code editors support it to my knowledge. Joplin has some hacks in place to make spellchecking work, but it results in a degraded editor, so the toggle is placed inside the options with a warning for now.
Eventually this shouldn't be an issue any more and the markdown editor component will just support spell checking.
