der: remove remaining `doc(cfg(...))` #1376

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	33s
build (1.75.0, wasm32-unknown-unknown)	32s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	31s
test (1.75.0)	1m 18s
test (stable)	1m 9s
	4m 26s

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