[DOC] Tweaks for StringIO#string= #558

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 2.7	28s
ubuntu-24.04 3.3	22s
ubuntu-24.04 3.1	14s
ubuntu-22.04 2.7	25s
ubuntu-24.04 3.0	15s
ubuntu-24.04 jruby-head	1m 6s
ubuntu-24.04 3.2	18s
ubuntu-22.04 3.0	12s
ubuntu-24.04 head	13s
ubuntu-22.04 3.2	24s
ubuntu-24.04 truffleruby-head	31s
ubuntu-22.04 3.1	23s
ubuntu-22.04 3.3	12s
ubuntu-24.04 debug	28s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-22.04 3.4	23s
ubuntu-22.04 debug	25s
ubuntu-22.04 truffleruby-head	31s
ubuntu-22.04 head	12s
ubuntu-24.04 3.4	14s
	8m 30s

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