[DOC] Tweaks for StringIO#closed? #501

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	14s
ubuntu-24.04 2.7	40s
ubuntu-24.04 3.3	16s
ubuntu-24.04 3.2	24s
ubuntu-24.04 3.1	18s
ubuntu-24.04 3.4	18s
ubuntu-24.04 debug	27s
ubuntu-24.04 truffleruby-head	27s
ubuntu-24.04 jruby-head	1m 3s
ubuntu-24.04 head	16s
ubuntu-22.04 3.2	22s
ubuntu-22.04 2.7	46s
ubuntu-22.04 3.1	18s
ubuntu-22.04 3.0	12s
ubuntu-22.04 truffleruby-head	27s
ubuntu-22.04 3.4	20s
ubuntu-22.04 3.3	22s
ubuntu-22.04 jruby-head	1m 3s
ubuntu-22.04 head	16s
ubuntu-22.04 debug	28s
	9m 1s

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