[DOC] Tweaks for StringIO.open #492

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 2.7	50s
ubuntu-24.04 3.2	34s
ubuntu-24.04 3.1	26s
ubuntu-24.04 3.4	18s
ubuntu-24.04 3.0	27s
ubuntu-22.04 3.0	24s
ubuntu-24.04 3.3	31s
ubuntu-24.04 head	26s
ubuntu-22.04 2.7	53s
ubuntu-22.04 debug	30s
ubuntu-24.04 jruby-head	1m 32s
ubuntu-24.04 truffleruby-head	39s
ubuntu-22.04 head	24s
ubuntu-22.04 jruby-head	1m 37s
ubuntu-22.04 3.3	27s
ubuntu-22.04 3.4	40s
ubuntu-24.04 debug	34s
ubuntu-22.04 truffleruby-head	39s
ubuntu-22.04 3.1	31s
ubuntu-22.04 3.2	32s
	12m 59s

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