Clarify the layout documentation for x86 SIMD types. #2363

Job	Run time
Check Style	19s
Automatic intrinsic verification	29s
Env Override (ubuntu-latest)	12s
Env Override (windows-latest)	25s
Build Documentation	1m 21s
Build std_detect	1m 3s
Test (i686-unknown-linux-gnu)	1m 58s
Test (x86_64-unknown-linux-gnu)	4m 6s
Test (x86_64-unknown-linux-gnu-emulated)	9m 34s
Test (arm-unknown-linux-gnueabihf)	1m 45s
Test (armv7-unknown-linux-gnueabihf)	10m 49s
Test (aarch64-unknown-linux-gnu)	21m 14s
Test (riscv64gc-unknown-linux-gnu)	1m 16s
Test (powerpc64le-unknown-linux-gnu)	3m 55s
Test (s390x-unknown-linux-gnu)	1m 23s
Test (wasm32-wasip1)	1m 39s
Test (i586-unknown-linux-gnu)	2m 17s
Test (nvptx64-nvidia-cuda)	26s
Test (thumbv6m-none-eabi)	23s
Test (thumbv7m-none-eabi)	18s
Test (thumbv7em-none-eabi)	21s
Test (thumbv7em-none-eabihf)	14s
Test (x86_64-apple-darwin)	7m 20s
Test (aarch64-apple-darwin)	56s
Test (x86_64-pc-windows-msvc)	6m 47s
Test (i686-pc-windows-msvc)	3m 6s
Test (aarch64-pc-windows-msvc)	1m 30s
Test (x86_64-pc-windows-gnu)	7m 7s
success	1s
	1h 32m 14s

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