[DOC] Doc for StringIO#each_char #476

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 7s
windows-latest jruby-head	2m 27s
windows-latest 3.2	1m 48s
windows-latest 3.3	1m 20s
windows-latest 3.1	1m 4s
windows-latest 2.7	1m 41s
windows-latest mingw	59s
windows-latest 3.4	1m 3s
windows-latest mswin	1m 7s
	12m 41s

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