[DOC] Tweaks for StringIO#fsync #511

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.2	1m 2s
windows-latest 3.1	59s
windows-latest 3.0	1m 10s
windows-latest 3.4	1m 4s
windows-latest jruby-head	1m 49s
windows-latest 2.7	1m 35s
windows-latest 3.3	1m 3s
windows-latest mingw	57s
windows-latest mswin	1m 9s
	10m 53s

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