Update the polyfill to the latest I/O safety documentation. #3374

Job	Run time
Check --no-default-features (nightly)	1m 12s
Rustfmt	10s
Check nightly-only targets (nightly)	37s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 40s
Check selected Tier 3 platforms (nightly)	5m 20s
Check (stable)	8m 17s
Check (nightly)	8m 21s
Test (ubuntu)	1m 2s
Check (1.63)	17m 37s
Test (ubuntu-20.04)	1m 2s
Test use-libc (ubuntu)	1m 4s
Test (i686-linux)	1m 46s
Test use-libc (i686-linux)	1m 39s
Test (aarch64-linux)	2m 15s
Test use-libc (aarch64-linux)	1m 39s
Test (powerpc64le-linux)	1m 31s
Test use-libc (powerpc64le-linux)	2m 4s
Test (riscv64-linux)	1m 29s
Test use-libc (riscv64-linux)	1m 41s
Test (s390x-linux)	1m 55s
Test use-libc (arm-linux)	1m 44s
Test (arm-linux)	1m 37s
Test (ubuntu-stable)	1m 12s
Test (i686-linux-stable)	1m 25s
Test (aarch64-linux-stable)	1m 45s
Test (riscv64-linux-stable)	1m 22s
Test (s390x-linux-stable)	1m 38s
Test (powerpc64le-linux-stable)	2m 15s
Test (arm-linux-stable)	1m 25s
Test (ubuntu-1.63)	5m 30s
Test (i686-linux-1.63)	4m 56s
Test (aarch64-linux-1.63)	5m 5s
Test (riscv64-linux-1.63)	5m 0s
Test (s390x-linux-1.63)	6m 26s
Test (powerpc64le-linux-1.63)	5m 7s
Test (arm-linux-1.63)	5m 4s
Test (macos-latest)	20m 25s
Test (macos-11)	1m 9s
Test (windows)	1m 21s
Test (windows-2019)	1m 13s
	2h 18m 0s

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