[DOC] Tweaks for StringIO.closed_read? #502

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	40s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.1	15s
ubuntu-24.04 3.2	17s
ubuntu-24.04 debug	22s
ubuntu-24.04 3.3	15s
ubuntu-22.04 3.0	14s
ubuntu-24.04 3.4	14s
ubuntu-22.04 2.7	46s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-24.04 truffleruby-head	24s
ubuntu-22.04 3.3	21s
ubuntu-24.04 head	17s
ubuntu-22.04 3.1	19s
ubuntu-22.04 3.4	19s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-22.04 debug	31s
ubuntu-22.04 3.2	20s
ubuntu-22.04 truffleruby-head	23s
ubuntu-22.04 head	19s
	8m 52s

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