Add empty impl blocks if they have documentation Fixes https://github.com/rust-lang/rust/issues/90866. The update for the test script is needed to count the number of impl blocks we have with only the struct. To be noted that with https://github.com/rust-lang/rust/pull/89676 merged, it wouldn't be needed (I don't know what is the status of it btw. cc ```@Mark-Simulacrum).``` It looks like this:  cc ```@jyn514``` r? ```@camelid```
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