Add a comment to explicit RUBY_FL_USER2 | RUBY_FL_USER3 #462

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	40s
ubuntu-24.04 3.0	10s
ubuntu-24.04 3.1	14s
ubuntu-24.04 3.2	12s
ubuntu-24.04 3.3	15s
ubuntu-24.04 3.4	13s
ubuntu-24.04 debug	24s
ubuntu-24.04 head	13s
ubuntu-24.04 jruby-head	1m 6s
ubuntu-24.04 truffleruby-head	28s
ubuntu-22.04 2.7	37s
ubuntu-22.04 3.0	12s
ubuntu-22.04 3.1	17s
ubuntu-22.04 3.2	27s
ubuntu-22.04 3.3	16s
ubuntu-22.04 3.4	15s
ubuntu-22.04 debug	17s
ubuntu-22.04 head	11s
ubuntu-22.04 jruby-head	1m 4s
ubuntu-22.04 truffleruby-head	28s
	8m 2s

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