der: document `RefToOwned` and `OwnedToRef` #3149

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	26s
build (1.72.0, wasm32-unknown-unknown)	37s
minimal-versions	0s
test (1.72.0)	1m 15s
build (stable, thumbv7em-none-eabi)	24s
test (stable)	1m 2s
build (stable, wasm32-unknown-unknown)	36s
	4m 20s

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