Rollup merge of #26982 - nham:orphan-explanations, r=Gankro

Rollup merge of #26982 - nham:orphan-explanations, r=Gankro

part of #24407

I'm not sure whether I should be trying to explain the general rule in the E0210 explanation or just point people to the RFC. However, if we go with the latter option I think that the RFC will need to be revised slightly, since it is not quite as gentle as I would like.

Also, the link to RFC 1023 is not the correct one (it should be https://github.com/rust-lang/rfcs/blob/master/text/1023-rebalancing-coherence.md), but the correct one is too long. I'm aware of @michaelsproul's PR https://github.com/rust-lang/rust/pull/26290 from awhile back, but it doesn't seem to be working. Has there not been a new snapshot yet?