add comment to size_t nUpdate = kInvalid #365

Job	Run time
cuda	2m 46s
lcg (centos7-lcg100-gcc10)	11m 31s
linux_ubuntu	5m 11s
lcg (centos7-lcg101-gcc11)	6m 51s
lcg (centos8-lcg100-gcc10)	7m 59s
lcg (centos8-lcg101-gcc11)	6m 16s
linux_ubuntu_extra (ubuntu2004, 17)	9m 38s
linux-nodeps (centos8-lcg100-gcc10)	25m 45s
linux_ubuntu_extra (ubuntu2204_cpp20, 20)	5m 54s
linux_ubuntu_extra (ubuntu2204_clang, 17)	6m 31s
macos	9m 2s
sycl	2m 59s
linux_physmon	30m 55s
linux_examples_test	24m 54s
linux_physmon_perf_report	0s
	2h 36m 12s

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