[DOC] Tweaks for StringIO#binmode #493

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.1	25s
ubuntu-24.04 3.2	34s
ubuntu-24.04 3.0	28s
ubuntu-24.04 3.3	30s
ubuntu-24.04 3.4	18s
ubuntu-24.04 2.7	57s
ubuntu-24.04 debug	30s
ubuntu-22.04 2.7	54s
ubuntu-24.04 head	22s
ubuntu-24.04 jruby-head	1m 38s
ubuntu-22.04 3.1	32s
ubuntu-24.04 truffleruby-head	42s
ubuntu-22.04 3.0	25s
ubuntu-22.04 head	25s
ubuntu-22.04 3.4	30s
ubuntu-22.04 3.3	30s
ubuntu-22.04 jruby-head	1m 35s
ubuntu-22.04 debug	32s
ubuntu-22.04 3.2	30s
ubuntu-22.04 truffleruby-head	41s
	13m 3s

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