[DOC] Tweaks for StringIO#flush #555

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.3	18s
ubuntu-24.04 3.1	11s
ubuntu-24.04 3.0	17s
ubuntu-24.04 3.2	21s
ubuntu-24.04 jruby-head	1m 12s
ubuntu-24.04 2.7	31s
ubuntu-22.04 3.0	18s
ubuntu-24.04 head	20s
ubuntu-24.04 3.4	17s
ubuntu-22.04 3.1	20s
ubuntu-22.04 3.2	20s
ubuntu-24.04 debug	22s
ubuntu-22.04 2.7	29s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-22.04 3.4	18s
ubuntu-22.04 debug	20s
ubuntu-22.04 3.3	12s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 head	12s
	8m 28s

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