Add short doc examples for str::from_utf8_mut

[smt923]

Fixes #44462

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 51bbd69 has been approved by alexcrichton

[kennytm]

Tidy check failed. The source code should not contain trailing whitespaces.

[00:03:55] tidy error: /checkout/src/libcore/str/mod.rs:321: trailing whitespace
[00:03:55] tidy error: /checkout/src/libcore/str/mod.rs:326: trailing whitespace
[00:03:57] some tidy checks failed

[frewsxcv]

@bors r+ rollup

thanks again!

[bors]

📌 Commit 204414b has been approved by frewsxcv