When a user enables syntax highlighting and opens an edit window for the first time, a popover appears, as seen here: https://wikimedia.invisionapp.com/share/25BK644SD#/232930653_default
Text -- NOTE: THIS IS DIFFERENT FROM THE MOCKUP
Syntax highlighting will help you easily distinguish article text from wikitext markup
Clicking the "Try it" button turns syntax highlighting on. Clicking "No, thank you" dismisses the popover.
As per conversation in T163919: Design review for CodeMirror extension