Auto merge of #136244 - yotamofek:pr/rustdoc-join-iter, r=GuillaumeGomez

librustdoc: create a helper for separating elements of an iterator instead of implementing it multiple times

This implements something similar to [`Itertools::format`](https://docs.rs/itertools/latest/itertools/trait.Itertools.html#method.format), but on `Fn`s returning iterators instead of directly on iterators, to allow implementing `Display` without the use of a `Cell` (to handle the possibility of `fmt` being called multiple times while receiving `&self`).

~This is WIP, I just want to get a perf run first to see if the regression I saw in #135494 is fixed~

This was originally part of #135494 , but originally caused a perf regression that was since fixed:
https://github.com/rust-lang/rust/blob/7d5ae1863aa66847a4edf8d2ef9420717df65c5d/src/librustdoc/html/format.rs#L507

Author: bors