[DOC] Tweaks for StringIO.closed_read? #508

Job	Run time
ruby-versions / Generate Ruby versions	6s
ubuntu-24.04 3.0	13s
ubuntu-24.04 3.1	15s
ubuntu-24.04 3.2	18s
ubuntu-24.04 3.4	13s
ubuntu-24.04 3.3	27s
ubuntu-24.04 head	16s
ubuntu-22.04 2.7	41s
ubuntu-22.04 3.3	26s
ubuntu-24.04 truffleruby-head	22s
ubuntu-22.04 3.2	20s
ubuntu-22.04 3.1	20s
ubuntu-24.04 2.7	47s
ubuntu-24.04 debug	27s
ubuntu-22.04 3.0	19s
ubuntu-22.04 truffleruby-head	27s
ubuntu-24.04 jruby-head	1m 3s
ubuntu-22.04 head	18s
ubuntu-22.04 jruby-head	1m 6s
ubuntu-22.04 3.4	22s
ubuntu-22.04 debug	29s
	9m 15s

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