[DOC] Tweaks for StringIO#pos #610

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	43s
ubuntu-24.04 3.3	25s
ubuntu-24.04 3.1	16s
ubuntu-24.04 3.2	17s
ubuntu-24.04 debug	22s
ubuntu-24.04 3.0	20s
ubuntu-24.04 3.4	22s
ubuntu-24.04 head	15s
ubuntu-24.04 truffleruby-head	33s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-22.04 3.0	14s
ubuntu-22.04 3.2	17s
ubuntu-22.04 3.1	12s
ubuntu-22.04 head	18s
ubuntu-22.04 3.4	19s
ubuntu-22.04 2.7	48s
ubuntu-22.04 3.3	16s
ubuntu-22.04 jruby-head	1m 12s
ubuntu-22.04 debug	26s
ubuntu-22.04 truffleruby-head	31s
	9m 17s

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