[DOC] Class doc for StringIO #520

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 5s
windows-latest 2.7	1m 30s
windows-latest 3.1	1m 6s
windows-latest 3.2	1m 2s
windows-latest 3.4	1m 9s
windows-latest jruby-head	1m 35s
windows-latest 3.3	56s
windows-latest mswin	1m 1s
windows-latest mingw	55s
	10m 24s

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