[DOC] Tweaks for StringIO#close_read #440

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 3.0	58s
windows-latest 3.4	56s
windows-latest 2.7	1m 31s
windows-latest jruby-head	1m 39s
windows-latest 3.2	1m 7s
windows-latest mingw	1m 44s
windows-latest mswin	1m 7s
windows-latest 3.1	1m 1s
windows-latest 3.3	1m 0s
	11m 6s

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