[DOC] Tweaks for StringIO#close_write #520

Job	Run time
ruby-versions / Generate Ruby versions	8s
ubuntu-24.04 3.0	14s
ubuntu-24.04 head	18s
ubuntu-24.04 2.7	42s
ubuntu-24.04 3.1	13s
ubuntu-24.04 truffleruby-head	24s
ubuntu-24.04 3.3	16s
ubuntu-22.04 2.7	43s
ubuntu-22.04 3.0	16s
ubuntu-24.04 3.2	22s
ubuntu-24.04 jruby-head	1m 2s
ubuntu-24.04 3.4	17s
ubuntu-22.04 3.2	20s
ubuntu-24.04 debug	24s
ubuntu-22.04 3.1	23s
ubuntu-22.04 3.4	19s
ubuntu-22.04 debug	28s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 head	14s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 3.3	15s
	8m 57s

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