Trailing blank line removal in doc comments is not idempotent

Description

Only one trailing, blank line in a comment is removed each time rustfmt is run, so formatting is not idempotent if multiple trailing, blank lines are present. Blank line removal is enabled by any of the normalize_comments, wrap_comments, or format_code_in_doc_comments (for doc comments only) configuration options. All these features are unstable, so this affects nightly only (as of 2024-05-21 / d5f1200ed6a8e375f963e0c59a8bee45c0018c55)

This issue is fixed by PR #6163.

Reproduction

Given

rustfmt.toml

wrap_comments = true

code.rs

//! I am a humble module doc comment with many trailing lines
//!
//!

Run

# rustfmt code.rs

Expected

code.rs

//! I am a humble module doc comment with many trailing lines

Actual

code.rs

//! I am a humble module doc comment with many trailing lines
//!

Start with the reproduction in rustfmt.toml and code.rs, using wrap_comments = true and running rustfmt on the module doc comment. Compare the result with the expected output and inspect PR #6163 for the existing fix; done means repeated formatting leaves no trailing blank comment lines and remains unchanged on the next run.