[DOC] Tweaks for StringIO#string= (#172) #566

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 3.0	17s
ubuntu-24.04 3.2	22s
ubuntu-24.04 2.7	27s
ubuntu-24.04 3.1	15s
ubuntu-22.04 2.7	28s
ubuntu-24.04 3.4	20s
ubuntu-22.04 3.0	14s
ubuntu-24.04 debug	23s
ubuntu-24.04 head	18s
ubuntu-24.04 truffleruby-head	27s
ubuntu-24.04 3.3	22s
ubuntu-22.04 3.1	22s
ubuntu-24.04 jruby-head	1m 9s
ubuntu-22.04 3.4	22s
ubuntu-22.04 3.3	14s
ubuntu-22.04 3.2	26s
ubuntu-22.04 truffleruby-head	27s
ubuntu-22.04 debug	25s
ubuntu-22.04 jruby-head	1m 7s
ubuntu-22.04 head	18s
	8m 46s

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