[DOC] Tweaks for StringIO#flush #496

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 2.7	1m 35s
windows-latest 3.4	56s
windows-latest 3.2	1m 4s
windows-latest 3.1	55s
windows-latest jruby-head	1m 40s
windows-latest mingw	50s
windows-latest 3.3	1m 10s
windows-latest mswin	1m 21s
windows-latest 3.0	58s
	10m 34s

Provide feedback