[DOC] Tweaks for StringIO#fileno #495

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.2	1m 8s
windows-latest mingw	1m 5s
windows-latest 3.0	1m 3s
windows-latest jruby-head	1m 51s
windows-latest 3.3	1m 14s
windows-latest 2.7	1m 36s
windows-latest 3.1	1m 5s
windows-latest 3.4	1m 25s
windows-latest mswin	1m 21s
	11m 52s

Provide feedback