Clarify docs for `formatNum` to indicate that the lack of the fourth parameter means the third parameter is treated as the number of significant digits.
Clarify docs for `formatNum` to indicate that the lack of the fourth parameter means the third parameter is treated as the number of significant digits.

PiperOrigin-RevId: 421926527

google/closure-templates,google/closure-templates,google/closure-templates,google/closure-templates,google/closure-templates