[DOC] Doc for StringIO#each_codepoint #537

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 3.2	23s
ubuntu-24.04 debug	28s
ubuntu-24.04 3.1	14s
ubuntu-24.04 3.4	16s
ubuntu-24.04 3.3	13s
ubuntu-24.04 2.7	43s
ubuntu-24.04 truffleruby-head	31s
ubuntu-22.04 2.7	46s
ubuntu-22.04 3.0	17s
ubuntu-24.04 3.0	17s
ubuntu-22.04 3.2	16s
ubuntu-24.04 head	18s
ubuntu-22.04 3.1	30s
ubuntu-22.04 truffleruby-head	31s
ubuntu-22.04 head	19s
ubuntu-22.04 3.4	20s
ubuntu-22.04 3.3	23s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-22.04 debug	26s
ubuntu-22.04 jruby-head	1m 15s
	9m 36s

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