der: remove remaining `doc(cfg(...))` #1429

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	28s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	32s
test (1.75.0)	2m 46s
build (stable, thumbv7em-none-eabi)	22s
test (stable)	2m 32s
build (stable, wasm32-unknown-unknown)	22s
	7m 2s

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