der: remove remaining `doc(cfg(...))` (#1518) #1373

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 58s
build (1.65.0, wasm32-unknown-unknown)	1m 45s
build (stable, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	18s
test (1.65.0)	2m 7s
test (stable)	28s
minimal-versions / minimal-versions	39s
	7m 31s

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