der: remove remaining `doc(cfg(...))` #3227

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	41s
build (1.72.0, wasm32-unknown-unknown)	36s
minimal-versions	0s
test (1.72.0)	4m 23s
build (stable, thumbv7em-none-eabi)	39s
test (stable)	4m 22s
build (stable, wasm32-unknown-unknown)	41s
	11m 22s

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