der: document `RefToOwned` and `OwnedToRef` #1347

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	32s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	29s
test (1.75.0)	3m 48s
build (stable, thumbv7em-none-eabi)	33s
build (stable, wasm32-unknown-unknown)	24s
test (stable)	3m 30s
	9m 16s

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