[DOC] Tweaks for StringIO#isatty #494

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.0	1m 24s
windows-latest 2.7	1m 40s
windows-latest jruby-head	1m 53s
windows-latest 3.1	1m 14s
windows-latest 3.4	1m 0s
windows-latest 3.3	1m 9s
windows-latest 3.2	1m 9s
windows-latest mingw	56s
windows-latest mswin	1m 5s
	11m 34s

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