[DOC] Doc for StringIO#each_codepoint #531

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	14s
ubuntu-24.04 2.7	44s
ubuntu-24.04 3.3	14s
ubuntu-24.04 3.2	24s
ubuntu-24.04 3.1	17s
ubuntu-24.04 head	20s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-24.04 debug	19s
ubuntu-22.04 2.7	48s
ubuntu-22.04 3.1	20s
ubuntu-22.04 3.3	14s
ubuntu-24.04 3.4	11s
ubuntu-22.04 3.0	20s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 debug	27s
ubuntu-22.04 3.2	20s
ubuntu-22.04 3.4	20s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 head	11s
ubuntu-22.04 truffleruby-head	30s
	9m 1s

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