syz-cluster: explain graphs on the stats page #17718

Job	Run time
aux	4m 35s
build	3m 15s
race	7m 34s
dashboard	2m 36s
old	6m 57s
arch (presubmit_arch_freebsd)	6m 11s
arch (presubmit_arch_darwin)	3m 45s
gvisor	3m 53s
arch (presubmit_arch_windows)	3m 30s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_openbsd)	3m 37s
race_dashboard	3m 57s
arch (presubmit_arch_netbsd)	3m 44s
arch (presubmit_arch_linux)	10m 19s
	1h 7m 9s

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