[DOC] Doc for StringIO#each #445

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.0	56s
windows-latest 2.7	1m 35s
windows-latest 3.1	1m 6s
windows-latest 3.3	1m 2s
windows-latest 3.2	1m 37s
windows-latest mingw	55s
windows-latest jruby-head	1m 33s
windows-latest mswin	1m 17s
windows-latest 3.4	1m 13s
	11m 18s

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