digest: remove lingering `docsrs` attrs #2253

Job	Run time
test (1.71.0)	22s
test (stable)	23s
build (1.71.0, thumbv7em-none-eabi)	17s
build (1.71.0, wasm32-unknown-unknown)	16s
build (stable, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	16s
	1m 50s

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