Document that if you use the Certbot snap, the plugins also need to be snapped

[alexzorin]

This is not at all obvious. Most users will have no idea about snap confinement.

I've seen a couple of community members hit this sticking point.

It would be preferable that they are armed with this knowledge upfront, rather than going through the experience of trying to get it working and giving up.

I think we should make this requirement visible on at least the wildcard tab of the snap instructions, if not both.