It is indeed tricky where to put this information.
For what it’s worth, I think we probably had the expectation that everything would auto-connect based on The content interface | Snapcraft documentation which says snaps from the same publisher will auto-connect without mentioning any caveats and later talks about how having multiple slots connected to the same plug is supported. Clarifying this across all interface docs is a lot of work though and may not even be desired to have all docs get bogged down in the details of that.
I think the biggest thing though is just having some some documentation somewhere aimed at developers/publishers of snaps that this is the expected, default behavior and ideally what your options for changing it are. I had to do a fair bit of searching before I came across Plug/slot declaration rules: greedy plugs and like I said, I struggled to understand it at least partially because I wasn’t the target audience.
You have that backwards. If you read my initial post again, you’ll see that the Certbot snap has the plug and is consuming the extra features added by many different slots.
I’ll look into making a store-requests post to change the behavior of the Certbot snap.