[DOC] Class doc for StringIO #597

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 debug	22s
ubuntu-24.04 2.7	44s
ubuntu-24.04 3.3	19s
ubuntu-24.04 3.4	20s
ubuntu-24.04 3.2	17s
ubuntu-24.04 3.1	12s
ubuntu-22.04 3.0	15s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-24.04 3.0	20s
ubuntu-22.04 3.2	20s
ubuntu-22.04 3.1	18s
ubuntu-22.04 3.4	23s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 2.7	49s
ubuntu-22.04 head	15s
ubuntu-24.04 head	17s
ubuntu-22.04 debug	29s
ubuntu-22.04 truffleruby-head	34s
ubuntu-22.04 3.3	18s
ubuntu-22.04 jruby-head	1m 15s
	9m 26s

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