pkcs5

der: document `RefToOwned` and `OwnedToRef` #3120

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	36s
build (1.72.0, wasm32-unknown-unknown)	36s
minimal-versions	0s
test (1.72.0)	3m 35s
build (stable, thumbv7em-none-eabi)	29s
test (stable)	3m 29s
build (stable, wasm32-unknown-unknown)	29s
	9m 14s

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