Fix some SIMD intrinsics documentation

[meesfrensel]

Spotted some mistakes in the docs of some SIMD intrinsics.

[rustbot]

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[rustbot]

Some changes occurred to the platform-builtins intrinsics. Make sure the
LLVM backend as well as portable-simd gets adapted for the changes.

cc @antoyo, @GuillaumeGomez, @bjorn3, @calebzulawski, @programmerjake

[calebzulawski]

Thanks!
@bors r+

[bors]

📌 Commit a85f6a6 has been approved by calebzulawski

It is now in the queue for this repository.