[DOC] Doc for StringIO#each_codepoint #532

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	51s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.2	23s
ubuntu-24.04 3.1	19s
ubuntu-24.04 3.4	14s
ubuntu-24.04 3.3	18s
ubuntu-22.04 3.0	14s
ubuntu-24.04 debug	22s
ubuntu-24.04 head	13s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 3.2	15s
ubuntu-24.04 jruby-head	1m 0s
ubuntu-22.04 3.4	28s
ubuntu-22.04 3.3	20s
ubuntu-22.04 3.1	24s
ubuntu-22.04 debug	22s
ubuntu-22.04 head	21s
ubuntu-22.04 truffleruby-head	37s
ubuntu-22.04 jruby-head	1m 13s
ubuntu-22.04 2.7	43s
	9m 22s

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