der: remove remaining `doc(cfg(...))` (#1518) #990

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	29s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	29s
build (stable, thumbv7em-none-eabi)	17s
test (1.75.0)	36s
build (stable, wasm32-unknown-unknown)	19s
test (stable)	22s
	2m 32s

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