[DOC] Tweaks for StringIO#close_read #499

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.1	13s
ubuntu-24.04 3.0	19s
ubuntu-24.04 3.2	37s
ubuntu-24.04 2.7	44s
ubuntu-24.04 debug	24s
ubuntu-24.04 head	16s
ubuntu-24.04 3.3	23s
ubuntu-22.04 3.1	25s
ubuntu-22.04 3.4	19s
ubuntu-22.04 debug	21s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-22.04 3.3	23s
ubuntu-24.04 3.4	16s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 2.7	45s
ubuntu-22.04 3.2	18s
ubuntu-22.04 3.0	19s
ubuntu-22.04 head	14s
ubuntu-22.04 jruby-head	1m 7s
ubuntu-22.04 truffleruby-head	32s
	9m 37s

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