der: document `RefToOwned` and `OwnedToRef` #1901

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	58s
build (1.75.0, wasm32-unknown-unknown)	59s
fuzz	2m 12s
build (stable, thumbv7em-none-eabi)	52s
test (1.75.0)	4m 46s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	50s
test (stable)	4m 36s
	15m 13s

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