[DOC] Tweaks for StringIO#eof? #482

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.4	1m 30s
windows-latest 2.7	2m 17s
windows-latest 3.3	1m 45s
windows-latest mingw	1m 37s
windows-latest jruby-head	2m 11s
windows-latest 3.1	1m 48s
windows-latest 3.2	1m 48s
windows-latest 3.0	1m 33s
windows-latest mswin	1m 42s
	16m 15s

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