Document movnt needs sfence

[workingjubilee]

For every intrinsic that may generate any of the MOVNT family of instructions, specify it must be followed by _mm_sfence.

Also, ask people to not think too hard about what actually happens with write-combining memory buffers. They probably don't want to know, and in terms of the Rust abstract machine, we aren't actually entirely sure yet.

[rustbot]

r? @Amanieu

(rustbot has picked a reviewer for you, use r? to override)

[workingjubilee]

Also see:

• https://github.com/rust-lang/rust/issues/114582
• https://github.com/llvm/llvm-project/issues/64521

[workingjubilee]

Will address the comments after the conversation settles a bit.

[bors]

:umbrella: The latest upstream changes (presumably c8ae80e8802c25fef106a5e605b2ccec57522dc9) made this pull request unmergeable. Please resolve the merge conflicts.