[DOC] Tweaks for StringIO#close_read #509

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	44s
ubuntu-24.04 3.3	25s
ubuntu-24.04 3.4	18s
ubuntu-24.04 3.2	19s
ubuntu-24.04 3.1	15s
ubuntu-24.04 3.0	16s
ubuntu-24.04 debug	24s
ubuntu-24.04 truffleruby-head	21s
ubuntu-24.04 jruby-head	58s
ubuntu-24.04 head	16s
ubuntu-22.04 2.7	40s
ubuntu-22.04 truffleruby-head	24s
ubuntu-22.04 3.2	22s
ubuntu-22.04 jruby-head	1m 1s
ubuntu-22.04 debug	20s
ubuntu-22.04 3.4	23s
ubuntu-22.04 head	20s
ubuntu-22.04 3.3	26s
ubuntu-22.04 3.1	20s
ubuntu-22.04 3.0	22s
	8m 58s

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