Switch from `doc_auto_cfg` to `doc_cfg` #923

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	16s
build (1.85.0, wasm32-unknown-unknown)	17s
test (1.85.0)	22s
build (stable, thumbv7em-none-eabi)	15s
test (stable)	28s
minimal-versions	0s
	1m 58s

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