[DOC] Tweaks for StringIO.closed_write? #503

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	16s
ubuntu-24.04 3.2	19s
ubuntu-24.04 3.3	21s
ubuntu-24.04 2.7	42s
ubuntu-24.04 debug	21s
ubuntu-24.04 3.4	13s
ubuntu-24.04 3.1	18s
ubuntu-24.04 truffleruby-head	28s
ubuntu-24.04 head	14s
ubuntu-24.04 jruby-head	1m 1s
ubuntu-22.04 3.0	17s
ubuntu-22.04 3.3	19s
ubuntu-22.04 3.4	20s
ubuntu-22.04 3.1	25s
ubuntu-22.04 2.7	41s
ubuntu-22.04 truffleruby-head	22s
ubuntu-22.04 head	14s
ubuntu-22.04 jruby-head	1m 3s
ubuntu-22.04 3.2	26s
ubuntu-22.04 debug	22s
	8m 46s

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