[DOC] Tweaks for StringIO#close_read #450

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 2.7	1m 33s
windows-latest 3.0	1m 1s
windows-latest 3.1	1m 1s
windows-latest 3.2	58s
windows-latest 3.3	1m 38s
windows-latest mingw	56s
windows-latest jruby-head	1m 41s
windows-latest mswin	1m 2s
windows-latest 3.4	1m 6s
	11m 1s

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