Document `pub upgrade` flag `--unlock-transitive` #1890

Job	Run time
Check if code excerpts are up to date	16s
Validate Firebase configuration	18s
Build site and check links	52s
Check if text can be replaced with site variables	19s
Analyze and test code examples (beta, false)	1m 57s
Analyze and test code examples (stable, false)	1m 53s
	5m 35s

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