[DOC] Doc for StringIO#each_codepoint #473

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 2.7	1m 41s
windows-latest 3.3	1m 5s
windows-latest 3.0	58s
windows-latest mingw	49s
windows-latest 3.4	59s
windows-latest mswin	1m 13s
windows-latest 3.2	1m 0s
windows-latest jruby-head	1m 36s
windows-latest 3.1	1m 9s
	10m 35s

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