[DOC] Tweaks for StringIO.closed_write? #511

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	43s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.2	23s
ubuntu-24.04 3.1	25s
ubuntu-24.04 head	17s
ubuntu-24.04 3.4	16s
ubuntu-24.04 jruby-head	1m 5s
ubuntu-24.04 debug	27s
ubuntu-24.04 3.3	17s
ubuntu-22.04 2.7	46s
ubuntu-22.04 3.3	20s
ubuntu-22.04 3.0	20s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 3.2	21s
ubuntu-24.04 truffleruby-head	23s
ubuntu-22.04 3.1	22s
ubuntu-22.04 3.4	22s
ubuntu-22.04 debug	26s
ubuntu-22.04 head	14s
ubuntu-22.04 truffleruby-head	23s
	9m 16s

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