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Merge pull request #2088 from tzz-radxa/docs/q900-download-os-clarify… #4213

Job	Run time
build	12m 34s
test-optional (gitown, manual)	43s
test (push)	43s
test (commit)	1m 1s
test-optional (check-external-links, manual)	56s
deploy	1m 41s
	17m 38s