der: remove remaining `doc(cfg(...))` #3106

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	36s
minimal-versions	0s
build (1.72.0, wasm32-unknown-unknown)	34s
test (1.72.0)	1m 21s
build (stable, thumbv7em-none-eabi)	25s
test (stable)	1m 11s
build (stable, wasm32-unknown-unknown)	31s
	4m 38s

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