While I did really enjoy those comment sections back in my PHP days with Elixir, just make PRs. José and co. are amazing about doc contributions. PRs are usually reviewed within a day. I’ve once had one accepted in under 20 mins from opening it! The big benefit here is that it gets vetted by the team and doesn’t have to rely on an upvoting system. José has even added to and reworded a PR I made to improve it. That particular contribution was largely centred around a gotcha much in the spirit of those PHP comments 
So even if you are unsure about what you’re contributing, it will get the ball rolling and others can help to fix it (or you can post it here first, of course).
Otherwise, hexdocs are static HTML so I think implementing a dynamic section that would require an upvote system would very likely be a non-starter.
