der: document `RefToOwned` and `OwnedToRef` #1315

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	27s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	27s
test (1.75.0)	1m 19s
build (stable, thumbv7em-none-eabi)	26s
test (stable)	1m 9s
build (stable, wasm32-unknown-unknown)	24s
	4m 12s

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