[DOC] Doc for StringIO#each_char #470

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 2.7	1m 47s
windows-latest 3.3	1m 7s
windows-latest 3.2	1m 14s
windows-latest 3.1	1m 1s
windows-latest mingw	59s
windows-latest 3.0	1m 7s
windows-latest mswin	1m 12s
windows-latest jruby-head	1m 45s
windows-latest 3.4	1m 6s
	11m 23s

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