https://github.com/flaki your understanding is correct. microformats2 (specifically microformats.org/wiki/h-entry for a https://indieweb.org/reply ) would work well to mark-up such useful asides and other notes / comments regarding MDN pages, notifying those pages using Webmention, and handling them accordingly via moderation etc.
How those asides or notes are displayed on the MDN page is up to the design of the MDN page.
In addition Webmention also allows for easily submitting updates to such asides, notes, and comments and having them handled automatically for accepted submissions, or again via a moderation step.