[DOC] Doc for StringIO#each_char #529

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 3.2	23s
ubuntu-24.04 3.4	18s
ubuntu-24.04 3.0	18s
ubuntu-24.04 3.3	13s
ubuntu-22.04 3.1	26s
ubuntu-24.04 2.7	48s
ubuntu-24.04 truffleruby-head	26s
ubuntu-24.04 debug	25s
ubuntu-22.04 3.3	18s
ubuntu-22.04 2.7	47s
ubuntu-24.04 head	12s
ubuntu-22.04 debug	22s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-22.04 3.2	27s
ubuntu-22.04 3.4	18s
ubuntu-22.04 head	13s
ubuntu-22.04 truffleruby-head	34s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 3.0	13s
ubuntu-24.04 3.1	13s
	9m 13s

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