Fix doc nits
Clippy is wonderful, and reading its lints is a great way to learn about Rust. While doing this, I noticed a few little copyedits, such as adding trailing periods to sentences, or tweaking wording very slightly to improve its readability. I hope you will accept these suggestions as an act of love for the project, with my thanks for all the maintainers' hard work.
changelog: Docs [ mut_range_bound ]: fix doc nits
changelog: Docs [ needless_for_each ]: fix doc nits
changelog: Docs [ arc_with_non_send_sync ]: fix doc nits
changelog: Docs [ allow_attributes ]: fix doc nits
changelog: Docs [ allow_attributes_without_reason ]: fix doc nits
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