zeroize: use `doc_auto_cfg` (#1063) #486

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	1m 32s
build (1.60.0, wasm32-unknown-unknown)	2m 26s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.60.0, sudo apt update && sudo apt install gcc-mult...	1m 53s
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	12s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.60.0)	1m 31s
test (x86_64-pc-windows-msvc, windows-latest, 1.60.0)	2m 0s
test (x86_64-apple-darwin, macos-latest, 1.60.0)	2m 4s
cross (aarch64-unknown-linux-gnu, 1.60.0)	1m 55s
cross (aarch64-unknown-linux-gnu, stable)	39s
cross (powerpc-unknown-linux-gnu, 1.60.0)	1m 58s
cross (powerpc-unknown-linux-gnu, stable)	32s
minimal-versions / minimal-versions	17s
	17m 14s

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