Document indexOf return value when not found #35288

Job	Run time
smoke	1m 2s
coverage	11m 1s
typecheck	39s
knip	32s
browser-integration	1m 54s
lint	51s
package-size	1m 32s
format	24s
misc	32s
Test Node 14 on windows-latest	14m 34s
baselines	9m 12s
Test Node 14 on ubuntu-latest	11m 37s
self-check	54s
Test Node lts/* on ubuntu-latest with --no-bundle	10m 22s
Test Node 24 on windows-latest	9m 49s
Test Node 24 on ubuntu-latest	8m 18s
required	3s
	1h 23m 16s

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