Add doc examples to str::from_utf8_unchecked_mut

[napen123]

Fixes#44461

[rust-highfive]

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[carols10cents]

r? @frewsxcv

Thank you for the PR!!

[frewsxcv]

it turns out that constructing an invalid string in an 'unchecked' manner is considered "undefined behavior" and not something we want to add in here. although it might not be obvious for such a simple example as this, this sort of behavior can cause the program to become unpredictable and problematic. unfortunately, i think we might just need to remove this "Invalid UTF-8" section for this example

[napen123]

Alright, I removed the Invalid UTF-8 section.

[frewsxcv]

thanks!

@bors r+ rollup

[bors]

📌 Commit 8adf50d has been approved by frewsxcv