der: improve `TeletexStringRef` docs (#1329) #2154

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	2m 18s
build (1.60.0, wasm32-unknown-unknown)	2m 22s
test (1.60.0)	2m 48s
build (stable, thumbv7em-none-eabi)	21s
test (stable)	24s
build (stable, wasm32-unknown-unknown)	14s
minimal-versions / minimal-versions	33s
	9m 0s

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