I think it’s the context that makes them especially tricksy in documentation. They do seem to occupy the natural space a ‘further reading’ section would be. Technically, they are tangentially related in so much that someone thought to link the guide in the connected topic. But it’s like having a ‘How to prepare an injection’ guide for your site - that could easily (and helpfully) be linked in every topic in reply to a new user finding their feet and looking for a starting place. However, you wouldn’t really want any new users arriving at the guide and then thinking they should work their way down those backlinks to all the times it’s been linked. 
I think that’s a good reason to hide that section on docs.
Backlinks can appear in a similar place on most posts if they’ve been linked elsewhere on the site. I think it’s often easier to tell what they are outside of the docs context, and because other posts aren’t generally linked so often so they don’t accumulate a 100+ list of them.
The Related Topics feature that’s part of the AI plugin is found at the bottom of a topic (when enabled), alongside the ‘suggested’ tab. That can be really good for finding similar topics, and has helped me out more times than I can count.