chore: fix TODO related to missing citation #3731

Job	Run time
Build source R package	5m 41s
Sanitizer	25m 51s
Coverage	1h 3m 8s
Check windows-latest (release)	11m 14s
Check ubuntu-20.04 (release)	6m 30s
Check ubuntu-20.04 (oldrel-1)	6m 43s
Check ubuntu-20.04 (oldrel-2)	7m 5s
Check ubuntu-20.04 (oldrel-3)	6m 34s
Check ubuntu-20.04 (oldrel-4)	6m 35s
	2h 19m 21s

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