[DOC] Tweaks for StringIO#pos= #552

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 2.7	1m 36s
windows-latest 3.0	1m 2s
windows-latest 3.2	1m 19s
windows-latest 3.1	1m 19s
windows-latest jruby-head	1m 36s
windows-latest 3.3	1m 11s
windows-latest mingw	54s
windows-latest mswin	1m 12s
windows-latest 3.4	52s
	11m 5s

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