Add safety sections to xrstor-family restore intrinsics

[kiana1kaslana]

_xrstor, _xrstors and their 64-bit variants are declared unsafe but carry no # Safety section. This adds the missing sections.

These instructions read back an XSAVE area written by a prior save, so the sections require mem_addr to be valid for reads of the whole area it describes and to be 64-byte aligned, and the memory to contain a valid XSAVE area matching the format in the referenced Intel SDM section.

Doc-only change.

[rustbot]

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

Why was this reviewer chosen?

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
• Random selection from Amanieu, adamgemmell, davidtwco, folkertdev, sayantn