pkcs5

der: improve `TeletexStringRef` docs #2786

Job	Run time
minimal-versions	0s
build (1.71.0, thumbv7em-none-eabi)	40s
build (1.71.0, wasm32-unknown-unknown)	44s
test (1.71.0)	1m 49s
build (stable, thumbv7em-none-eabi)	40s
test (stable)	1m 32s
build (stable, wasm32-unknown-unknown)	34s
	5m 59s

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