der: document `RefToOwned` and `OwnedToRef` #3020

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	1m 35s
derive (1.71.0)	22s
build (1.71.0, wasm32-unknown-unknown)	1m 30s
derive (stable)	15s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	1m 36s
build (stable, wasm32-unknown-unknown)	1m 24s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	11m 18s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	9m 48s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	10m 31s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	9m 53s
	48m 12s

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