do_blocks(): Document transient-memory-leak optimization. #23851

Job	Run time
Determine Matrix	5s
Multisite Memcached / Test current	13m 30s
Multisite Default / Test before	13m 20s
Multisite Default / Test current	13m 29s
Multisite Memcached / Test before	13m 20s
Single Site Default / Test current	14m 7s
Single Site Default / Test before	13m 58s
Single Site Memcached / Test current	13m 25s
Single Site Memcached / Test before	14m 23s
Compare / Single Site Default	13s
Compare / Single Site Memcached	12s
Compare / Multisite Memcached	12s
Compare / Multisite Default	18s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 50m 32s

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