[DOC] Class doc for StringIO (#178) #539

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 3.1	1m 5s
windows-latest 2.7	1m 39s
windows-latest jruby-head	2m 16s
windows-latest 3.2	1m 5s
windows-latest 3.0	1m 5s
windows-latest mswin	1m 15s
windows-latest 3.4	57s
windows-latest mingw	54s
windows-latest 3.3	55s
	11m 14s

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