der: remove remaining `doc(cfg(...))` #3196

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	25s
build (1.72.0, wasm32-unknown-unknown)	24s
test (1.72.0)	38s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	38s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions	0s
	2m 43s

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