Add safety sections to bittest intrinsics

[kiana1kaslana]

The _bittest family and their 64-bit variants are declared unsafe but carry no # Safety section. This adds the missing sections.

_bittest and _bittest64 only read the addressed integer, so they require p to be valid for reads of one integer. The bittestand* variants also modify it, so they require validity for reads and writes. All of them require b to stay within the width of the pointed-to integer.

Doc-only change.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @folkertdev (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

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The reviewer was selected based on:

• Owners of files modified in this PR: @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn
• @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn expanded to Amanieu, adamgemmell, davidtwco, folkertdev, sayantn
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