State that special-case `num` typing rules do not apply to extension type members

[lrhn]

The special-case rules for typing, e.g, int + int does not apply if + denotes an extension type member, even if the receiver is a subtype of int.

[eernstg]

This one has had an LGTM for a long time. I just closed a couple of discussion threads, and I don't think there's anything controversial in the remaining ones. So let's get it landed.