This guide specifically: https://guides.github.com/features/mastering-markdown/
It doesn't mention the <details><summary>..</summary>....</details> construct, which can sometimes be very useful.
I'm not certain that this is the best place to post suggestions.
The Mastering Markdown guide isn't intended to be a complete guide to Markdown. It's intended to be a guide that introduces the most commonly used syntax elements from Markdown and also include the GitHub-specific extensions that GitHub-flavored Markdown offers.
Also, because `<details>` is HTML and not Markdown, I'm not certain that a guide for teaching how Markdown works is the best place to introduce the rigors of how HTML works and must be formatted.
However, you are correct that `<details>` blocks can be very useful. I'll pass it along to the documentation team and see if they can figure out something that will work, even if it ends up in another guide.
Thanks for the feedback!