[DOC] Tweaks for StringIO#pos= (#194) #558

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.2	1m 1s
windows-latest 3.0	1m 4s
windows-latest 2.7	1m 37s
windows-latest 3.1	1m 16s
windows-latest 3.4	55s
windows-latest 3.3	57s
windows-latest jruby-head	1m 38s
windows-latest mingw	59s
windows-latest mswin	1m 3s
	10m 34s

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