der: document `RefToOwned` and `OwnedToRef` #3168

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	27s
build (1.72.0, wasm32-unknown-unknown)	30s
build (stable, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions	0s
test (1.72.0)	36s
test (stable)	34s
	2m 44s

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