[DOC] Tweaks for StringIO#binmode #434

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 52s
windows-latest 2.7	2m 18s
windows-latest 3.1	3m 49s
windows-latest jruby-head	2m 9s
windows-latest 3.4	3m 24s
windows-latest 3.2	1m 38s
windows-latest mingw	1m 22s
windows-latest 3.3	1m 20s
windows-latest mswin	1m 18s
	19m 15s

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