normalize_doc_attributes + wrap_comments can leave trailing whitespace

Summary

I tried to format this code:

// rustfmt-normalize_doc_attributes: true
// rustfmt-wrap_comments: true

pub trait ClientSystemAlertsExt {
    #[doc = "If this is included, only alerts created at or after this time will be returned.\n```ignore\nx\n```"]
    fn alert_list(&self) -> builder::AlertList<'_>;
}

Expected behavior

No trailing whitespace in any line (and no changes after running rustfmt on the formatted code i.e. idempotency).

Actual behavior

Trailing whitespace after " returned."

Configuration

As noted above: normalize_doc_attributes + wrap_comments

Start by reproducing the issue with the Rust trait and the normalize_doc_attributes and wrap_comments options shown in the report. Trace how the documentation string is normalized and wrapped; the work is done when no line has trailing whitespace and formatting the result again makes no changes.