compiler: Sort and doc ExternAbi variants
My personal brainworms found this ordering made the most sense while writing the CanonAbi PR. It is *an* ordering, at least, unlike the current mess. There has been no particular reason for the previous order ever since rust-lang/rust#136901, despite the comment I delete here. I just didn't change it.
Because I feel weird just fussing with variant ordering in the source definition, I also documented a bunch to the best of my ability.
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