[DOC] Tweaks for StringIO#close_write #515

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.1	13s
ubuntu-24.04 2.7	44s
ubuntu-24.04 3.4	21s
ubuntu-24.04 debug	23s
ubuntu-24.04 3.0	13s
ubuntu-24.04 head	16s
ubuntu-22.04 3.2	22s
ubuntu-22.04 3.1	19s
ubuntu-24.04 jruby-head	59s
ubuntu-22.04 3.0	17s
ubuntu-22.04 3.4	19s
ubuntu-22.04 2.7	45s
ubuntu-24.04 3.3	16s
ubuntu-22.04 3.3	15s
ubuntu-22.04 truffleruby-head	28s
ubuntu-24.04 truffleruby-head	22s
ubuntu-22.04 jruby-head	1m 2s
ubuntu-22.04 debug	27s
ubuntu-24.04 3.2	15s
ubuntu-22.04 head	18s
	8m 38s

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