rustdoc: use descriptive tooltip if doctest is conditionally ignored

[lolbinarycat]

fixes #141092

here's what it looks like now:

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[yotamofek]

Consider maybe doing something like this:

if tooltip != Tooltip::None {
    let tooltip = fmt::from_fn(|f| match &tooltip {
        Tooltip::IgnoreAll => f.write_str("This example is not tested"),
        Tooltip::IgnoreSome(platforms) => {
            f.write_str("This example is not tested on ")?;
            match &platforms[..] {
                [] => unreachable!(),
                [platform] => f.write_str(platform)?,
                [first, second] => write!(f, "{first} or {second}")?,
                [platforms @ .., last] => {
                    for platform in platforms {
                        write!(f, "{platform}, ")?;
                    }
                    write!(f, "or {last}")?;
                }
            }
            Ok(())
        }
        Tooltip::CompileFail => f.write_str("This example deliberately fails to compile"),
        Tooltip::ShouldPanic => f.write_str("This example panics"),
        Tooltip::Edition(edition) => write!(f, "This example runs with edition {edition}"),
        Tooltip::None => unreachable!(),
    });
    write_str(out, format_args!("<a href=\"#\" class=\"tooltip\" title=\"{tooltip}\">ⓘ</a>"));
}

• If you're already introducing a fmt::from_fn call, you can get rid of the tmp buffer and allocating to it (and that comment about lifetime extension is IMHO a bit redundant anyways)
• Slice patterns allow for slightly nicer handling of printing that list and the oxford comma means we only need to special-case the last platform given more than 2 platforms.