der: remove remaining `doc(cfg(...))` #3177

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	28s
build (1.72.0, wasm32-unknown-unknown)	30s
test (1.72.0)	1m 11s
build (stable, thumbv7em-none-eabi)	32s
test (stable)	1m 1s
build (stable, wasm32-unknown-unknown)	23s
minimal-versions	0s
	4m 5s

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