intra-doc link failed linkchecker when depending on `cfg(doc)`

[scottmcm]

In one version of #102169 I was using cfg(doc) to bring in a use that one of the intra-doc links needed: 96314f9

But which that worked to make x.py doc pass, it later failed linkcheck: https://github.com/rust-lang/rust/actions/runs/3111111451/jobs/5043028360

std/collections/struct.VecDeque.html:317: broken intra-doc link - [<code>mem::forget</code>]
std/collections/vec_deque/struct.VecDeque.html:317: broken intra-doc link - [<code>mem::forget</code>]

So I had to go back to a use with an allow (f0dc359) to get it to work again.