Arbitrary self types v2: dev guide updates.

[adetaylor]

[Kobzol]

Sorry, due to me messing up a git operation, we sadly had to force-push the whole commit history of rustc-dev-guide :( If you'd like to update this pull request, you will have to rebase it in a special way onto the new commit history (the new master):

git fetch origin --all
git checkout <pr-branch>
git rebase --onto origin/master origin/master-old
git push --force-with-lease

More context can be found here.