der: document `RefToOwned` and `OwnedToRef` #1440

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	30s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	25s
test (1.75.0)	1m 9s
build (stable, thumbv7em-none-eabi)	21s
test (stable)	1m 13s
build (stable, wasm32-unknown-unknown)	20s
	3m 58s

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