Documentation uses HRTB lifetime before declaring it

The [`str::rsplit_once`](https://doc.rust-lang.org/nightly/std/primitive.str.html#method.rsplit_once) docs say `<P as Pattern>::Searcher<'a>: for<'a> ReverseSearcher<'a>` where the code it's documenting says `for<'a> P::Searcher<'a>: ReverseSearcher<'a>`.

This almost certainly shows up in other places but this is where I first noticed it.