Document `pub upgrade` flag `--unlock-transitive` #1905

Job	Run time
Validate Firebase configuration	16s
Check if code excerpts are up to date	16s
Build site and check links	55s
Check if text can be replaced with site variables	22s
Analyze and test code examples (beta, false)	1m 47s
Analyze and test code examples (stable, false)	1m 50s
	5m 26s

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