* Updated the explanations around lifetimes a bit.
* Made sure the examples that should fail still fail in edition 2018.
* Prefer `rust,compile_fail` instead of `rust,ignore` ‒ the latter
allows the user to click on button and see the actual compile errors.
Also, this'll tell us if something stops failing.
This reverts commit 7f1d1c6d9a7be5e427bace30e740b16b25f25c92.
The original commit was created because mdBook and rustdoc had
different generation algorithms for header links; now with
https://github.com/rust-lang/rust/pull/39966 , the algorithms
are the same. So let's undo this change.
... when I came across this problem, I said "eh, this isn't fun,
but it doesn't take that long." I probably should have just actually
taken the time to fix upstream, given that they were amenable. Oh
well!