[DOC] Class doc for StringIO #538

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 2s
windows-latest 3.2	1m 6s
windows-latest 3.1	1m 14s
windows-latest 2.7	1m 34s
windows-latest 3.3	1m 43s
windows-latest mswin	1m 20s
windows-latest jruby-head	2m 12s
windows-latest mingw	55s
windows-latest 3.4	58s
	12m 9s

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