chore: cargo +nightly fmt with wrap_comments = true

[keroro520]

Commits:

• 2c39c0a Execute cargo +nightly fmt --all with option wrap_comments = true, to wrap some long code comments.
• 21b6367 The default wrapping format is mess for some comments; I adjust these manurally.

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I did not update rustfmt.toml for the below reasons:

• wrapping comments mess some math formulas and doc links;
• wrap_comments is unstable yet; it requires nightly Rust;
• wrapping comments is a less important task.

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Ref:

• docs: document errors #2456 (comment)
• https://rust-lang.github.io/rustfmt/?version=master&search=#wrap_comments
• https://rust-lang.github.io/rustfmt/?version=master&search=#comment_width
• [unstable option] wrap_comments rust-lang/rustfmt#3347

[keroro520]

I am not sure whether wrapping comments is meaningful. Expect response.

[doitian]

It has little benefits and is easy to conflict other PRs.

[keroro520]

Less benefit