der: remove remaining `doc(cfg(...))` #3048

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	1m 36s
derive (1.71.0)	22s
build (1.71.0, wasm32-unknown-unknown)	1m 31s
derive (stable)	16s
build (stable, thumbv7em-none-eabi)	1m 32s
minimal-versions	0s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	10m 52s
build (stable, wasm32-unknown-unknown)	1m 19s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	10m 13s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	10m 26s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	9m 59s
	48m 6s

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