Self-document the XDG_STATE_HOME fallback #441

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (3.1)	40s
test (3.3)	36s
test (head)	35s
test (3.4)	42s
test (3.2)	37s
test (3.0)	35s
test (debug)	53s
	4m 41s

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