zeroize: use `doc_auto_cfg` #621

Job	Run time
build (1.72.0, armv7a-none-eabi)	15s
aarch64 (aarch64-unknown-linux-gnu, 1.72.0)	46s
build (1.72.0, thumbv7em-none-eabi)	19s
cross (powerpc-unknown-linux-gnu, 1.72.0)	42s
aarch64 (aarch64-unknown-linux-gnu, stable)	41s
cross (powerpc-unknown-linux-gnu, stable)	43s
build (1.72.0, wasm32-unknown-unknown)	14s
build (stable, armv7a-none-eabi)	15s
build (stable, thumbv7em-none-eabi)	8s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.72.0, sudo apt update && sudo apt install gcc-mult...	39s
build (stable, wasm32-unknown-unknown)	9s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	42s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.72.0)	23s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	14s
test (x86_64-apple-darwin, macos-latest, 1.72.0)	27s
test (x86_64-apple-darwin, macos-latest, stable)	19s
test (x86_64-pc-windows-msvc, windows-latest, 1.72.0)	55s
test (x86_64-pc-windows-msvc, windows-latest, stable)	57s
minimal-versions / minimal-versions	48s
	9m 36s

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