der: improve `TeletexStringRef` docs #965

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	26s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	36s
build (stable, thumbv7em-none-eabi)	30s
test (1.75.0)	1m 41s
build (stable, wasm32-unknown-unknown)	23s
test (stable)	1m 35s
	5m 11s

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