[DOC] Tweaks for StringIO#close_write #521

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 2.7	47s
ubuntu-24.04 3.0	17s
ubuntu-24.04 3.2	28s
ubuntu-22.04 2.7	47s
ubuntu-24.04 3.4	12s
ubuntu-24.04 3.3	12s
ubuntu-22.04 3.0	12s
ubuntu-24.04 debug	28s
ubuntu-24.04 head	18s
ubuntu-24.04 truffleruby-head	27s
ubuntu-24.04 jruby-head	1m 4s
ubuntu-24.04 3.1	14s
ubuntu-22.04 3.2	21s
ubuntu-22.04 3.3	16s
ubuntu-22.04 debug	23s
ubuntu-22.04 3.1	24s
ubuntu-22.04 jruby-head	1m 13s
ubuntu-22.04 head	13s
ubuntu-22.04 truffleruby-head	31s
ubuntu-22.04 3.4	26s
	9m 18s

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