der: document `RefToOwned` and `OwnedToRef` #3199

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	41s
build (1.72.0, wasm32-unknown-unknown)	39s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	51s
build (stable, wasm32-unknown-unknown)	43s
test (1.72.0)	4m 26s
test (stable)	4m 36s
	11m 56s

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