[DOC] Doc for StringIO#each_codepoint (#159) #538

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 2.7	42s
ubuntu-24.04 3.3	14s
ubuntu-24.04 head	12s
ubuntu-24.04 jruby-head	1m 0s
ubuntu-24.04 3.2	18s
ubuntu-22.04 2.7	47s
ubuntu-24.04 truffleruby-head	26s
ubuntu-24.04 debug	23s
ubuntu-24.04 3.1	13s
ubuntu-22.04 truffleruby-head	33s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.4	17s
ubuntu-22.04 3.1	24s
ubuntu-22.04 debug	26s
ubuntu-22.04 3.0	18s
ubuntu-22.04 head	16s
ubuntu-22.04 3.3	16s
ubuntu-22.04 3.2	17s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 3.4	23s
	8m 53s

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