der: remove remaining `doc(cfg(...))` #1408

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	27s
build (1.75.0, wasm32-unknown-unknown)	26s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	27s
minimal-versions	0s
test (1.75.0)	3m 44s
test (stable)	3m 32s
	8m 59s

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