Document charAt edge case behavior #35594

Job	Run time
browser-integration	2m 9s
coverage	11m 58s
knip	1m 21s
typecheck	1m 27s
self-check	1m 37s
misc	1m 12s
format	1m 12s
package-size	2m 21s
lint	1m 38s
baselines	10m 9s
smoke	1m 48s
Test Node 14 on ubuntu-latest	12m 52s
Test Node 14 on windows-latest	1m 6s
Test Node 24 on windows-latest	57s
Test Node 24 on ubuntu-latest	9m 35s
Test Node lts/* on ubuntu-latest with --no-bundle	11m 40s
required	4s
	1h 13m 6s

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