der: remove remaining `doc(cfg(...))` #1962

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	57s
build (1.75.0, wasm32-unknown-unknown)	1m 2s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	57s
fuzz	2m 12s
build (stable, wasm32-unknown-unknown)	53s
test (1.75.0)	4m 53s
test (stable)	4m 33s
	15m 27s

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