[DOC] Tweaks for StringIO#fsync #570

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	25s
ubuntu-24.04 3.1	17s
ubuntu-24.04 3.0	17s
ubuntu-24.04 3.2	16s
ubuntu-24.04 head	15s
ubuntu-24.04 debug	23s
ubuntu-24.04 3.3	13s
ubuntu-24.04 truffleruby-head	33s
ubuntu-22.04 3.0	14s
ubuntu-22.04 3.2	22s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-24.04 3.4	22s
ubuntu-22.04 debug	25s
ubuntu-22.04 3.1	18s
ubuntu-22.04 3.3	16s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-22.04 2.7	30s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 head	15s
ubuntu-22.04 3.4	21s
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 3.0	17s
ubuntu-24.04 3.1	17s
ubuntu-24.04 3.4	22s
ubuntu-24.04 head	15s
ubuntu-24.04 truffleruby-head	33s
ubuntu-24.04 debug	23s
ubuntu-24.04 3.3	13s
ubuntu-24.04 3.2	16s
ubuntu-22.04 3.0	14s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-22.04 2.7	44s
ubuntu-22.04 debug	25s
ubuntu-24.04 2.7	50s
ubuntu-22.04 3.1	18s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 3.3	16s
ubuntu-22.04 head	15s
ubuntu-22.04 3.4	21s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-22.04 3.2	22s
	17m 39s

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