[DOC] Class doc for StringIO (#178) #598

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	48s
ubuntu-24.04 3.0	21s
ubuntu-24.04 3.2	21s
ubuntu-24.04 3.4	17s
ubuntu-24.04 3.3	18s
ubuntu-24.04 debug	30s
ubuntu-24.04 3.1	19s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 3.0	13s
ubuntu-24.04 head	19s
ubuntu-22.04 3.1	18s
ubuntu-22.04 2.7	42s
ubuntu-22.04 3.2	19s
ubuntu-22.04 debug	35s
ubuntu-24.04 jruby-head	1m 13s
ubuntu-22.04 3.4	15s
ubuntu-22.04 jruby-head	1m 14s
ubuntu-22.04 3.3	17s
ubuntu-22.04 truffleruby-head	33s
ubuntu-22.04 head	18s
	9m 39s

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