[DOC] Tweaks for StringIO#closed? #442

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.1	59s
windows-latest 3.0	1m 6s
windows-latest 2.7	1m 34s
windows-latest mingw	56s
windows-latest 3.2	55s
windows-latest 3.3	1m 2s
windows-latest 3.4	1m 41s
windows-latest mswin	1m 20s
windows-latest jruby-head	1m 36s
	11m 13s

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